Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Friday, 13 January 2017

Pearl gemstone

What is Pearl Gemstone?

A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Just like the shell of a clam, a pearl is composed of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes (baroque pearls) occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries. Because of this, pearl has become a metaphor for something rare, fine, admirable and valuable.
The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those currently sold. Imitation pearls are also widely sold in inexpensive jewellery, but the quality of their iridescence is usually very poor and is easily distinguished from that of genuine pearls. Pearls have been harvested and cultivated primarily for use in jewellery, but in the past were also used to adorn clothing. They have also been crushed and used in cosmetics, medicines and paint formulations.
Whether wild or cultured, gem-quality pearls are almost always nacreous and iridescent, like the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls (technically "calcareous concretions") of lesser shine or less spherical shape.

History and Introduction

Pearls are organic gemstones that are formed by shelled molluscs; mainly bivalved oysters and mussels. Pearls are made up of nacre (mother-of-pearl) which is mostly aragonite (calcium carbonate) and conchiolin (complex proteins that form mollusc shells). The aragonite microcrystals build up around an irritant. The name "pearl" is said to have originated from the Middle English word "perle", which in turn came from the Latin word "perna", meaning "leg", thought to be due to the ham-leg shape of the bivalve mollusc.
Natural pearls are extremely rare, incredibly expensive and typically small. Therefore people have developed ways to culture pearls, so that these beautiful gemstones can be enjoyed by many. In cultured pearls, some tissue or a mother-of-pearl bead is introduced into the mollusc shell. If successful, this process induces the animal to form a "pearl sac" whose cells secrete a layer of brownish protein called conchiolin over the irritant. This is followed by the secretion of numerous mineral layers of nacre composed of calcium carbonate in thin overlapping plates. Pearls can come from fresh or seawater molluscs. The beauty of pearls is that they can be plucked from the shell naturally beautiful, fully-formed and displaying perfect lustre with no need to be cut or polished.
The oldest recorded reference to pearls in history is the 7,500 year-old "Umm Al Quwain Pearl", which was found in a grave in a place that is now known as the UAE. Before pearls were cultured by man, they were harvested from the Persian Gulf, Sri Lankan waters, fresh water sources in China and the rivers of Europe. Later, Christopher Columbus discovered pearls in South America. When these natural pearls were almost depleted in the early twentieth century, the Chinese and Japanese began to culture pearls and the rest is history. Nowadays, pearls remain a treasured adornment and are cultured all around the world.

Physical properties

The unique luster of pearls depends upon the reflection, refraction, and diffraction of light from the translucent layers. The thinner and more numerous the layers in the pearl, the finer the luster. The iridescence that pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface. In addition, pearls (especially cultured freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black. The very best pearls have a metallic mirror-like luster.
Because pearls are made primarily of calcium carbonate, they can be dissolved in vinegar. Calcium carbonate is susceptible to even a weak acid solution because the crystals of calcium carbonate react with the acetic acid in the vinegar to form calcium acetate and carbon dioxide.

Identifying Pearl

Pearls can be identified by their pearly lustre and when rubbed gently against the teeth, a slightly rough surface texture can be detected, whereas imitation pearls feel smooth. The surface appearance of natural and cultured pearls is the same, but the density of cultured pearls is usually higher, at approximately 2.73 for most. The only definite way of distinguishing between natural and cultured pearls is to examine their internal structure. Experts use endoscopes to do this through the drill holes of pearls. Natural pearls have concentric internal layers.

Pearl: Origin and Gemstone Sources

Pearls are found and cultured in waters all over the world. Natural sea pearls are found in Australia, Japan, Central America, the Persian Gulf, the Gulf of Manaar (between India and Sri Lanka), the coast of Madagascar, Burma (Myanmar), the Philippines, the South Pacific Islands (including Tahiti and Fiji) and South America.
Natural river pearls are found in Asia, Europe and North America.
Cultured seawater pearl sources include Southeast Asia (such as Indonesia and the Philippines), Australia, China, French Polynesia, Japan, South Pacific Islands (including Tahiti and Fiji) and the Philippines.
Cultured freshwater pearl sources include China and Japan.
Basra pearls where basra moti in the Indian language, natural pearl gemstone which are found in the Indian Ocean are also called as basra pearls.

Freshwater and saltwater pearls

Freshwater and saltwater pearls may sometimes look quite similar, but they come from different sources.
Freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland (where they are protected under law). Most freshwater cultured pearls sold today come from China.
Saltwater pearls grow within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.

Determining Pearl Gemstone Value

Pearl Colour

Pearls are available in a variety of colours. Colours of pearls include the following: White, pink, silver, cream, golden, green, blue and black. Some pearls exhibit iridescence, which is known as orient.

Pearl Clarity and Lustre

The lustre of pearls depends on the quality of the nacre. Pearls should have the characteristic shiny pearly lustre and their surface should show sharp and bright reflections. The surfaces of good quality pearls are smooth and blemish-free with a suitable layer of nacre to increase durability.

Pearl Cut and Shape

Pearls can be round, oval, pear-shaped or misshapen (baroque pearls). However, the best materials are regularly shaped. The most valuable shapes are symmetrical spheres or symmetrical drops. Pearl sizes depend on the type of mollusc that they form in and they typically range from 2 mm to 16 mm in diameter.

Pearl Treatment

Pearls are often bleached to lighten and enhance their colour. In this way, a uniform colour can be achieved for beaded necklaces.

Metaphysical properties of Pearls

Pearls symbolise Purity, Spiritual Transformation, Charity, Honesty, Wisdom and Integrity, all the best within us. Pearls provide a clear vehicle for the advancing states of wisdom, as well as a clean channel for receipt of spiritual guidance.
Pearls can stimulate your femininity and help with self acceptance. They lift your spirits and make you feel calm and beautiful. They remind us to walk with Dignity.
Pearls not only provide a mirror in which to see ourselves, but give us insight into how we appear to others.
The ragged, rough grain of sand, transformed over time slowly growing into a object of great value and beauty. With it's humble beginnings, Pearls symbolise innocence and a pure heart, and help us get in touch with the simple honest things of life.
Pearl grants to its wearer extraordinary foresight, protects him from risky commercial transactions and from evil eye. Pearl's magical properties depend on its colour: white Pearl brings freedom, brownish - wisdom, yellowish - wealth, greenish - happiness.
Pearl signifies faith, charity and innocence. It enhances personal integrity and helps to provide a focus to ones attention. Pearl symbolises purity and is known as a “stone of sincerity”. It brings truth to situations and loyalty to a “cause”. Inhibits boisterous behaviour.

Healing properties of Pearls

Pearls have been used throughout medical history to help treat disorders of the digestive tract, muscular systems and the skin. Powders and potions containing pearls have also been developed to aid in fertility, as well as in easing the discomforts of the birthing process. Many cultures and societies have made similar claims for this lustrous gem.
The Chinese, for example, have depended on the healing properties of the pearl for centuries. They have used it to treat everything from simple eye ailments to serious heart problems, bleeding, fever and indigestion. To this day, the Chinese still use pearl powder as a skin whitener and a cosmetic, as do many other people around the world. In fact, the pearl is widely considered to be effective in controlling the skin conditions rosacea and acne.
Pearls are considered to be very effective in treating health issues related to the stomach, heart and spleen. It calms the mind and prevents hysteria, promoting mental stability.It was believed that Pearl boosts sexual energy, strengthens visual acuity and heals ears diseases. It is beneficial for heart, lungs (against tuberculosis, asthma, chronic bronchitis), kidneys, urinary system and liver. Pearl possesses sedative and also laxative effect, neutralises poison, lowers acidity, It relieves conditions of bloating and biliousness.
Pearl increases fertility and eases childbirth. Pearl also raises sensuality. Pearl was believed to be the main ingredient of "immortality elixir".

Properties of Pearl gemstone

ColourWhite, Blue, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Multicolored
Hardness2.5 - 4.5
Crystal SystemAmorphous
Refractive Index1.52 - 1.69
SG2.6 - 2.8
TransparencyOpaque
Double Refraction.156
LusterPearly
CleavageNone
Mineral ClassCalcium carbonate and conchiolin (organic) combined with water

Orthoclase gemstone

What is Orthoclase gemstone?

Orthoclase is a mineral that belongs to the family of silicate minerals called feldspar. The deference between the feldspars and the other silicates is that the former contain a mixture of the following chemical elements: Potassium K, Calcium Ca and Sodium Na, in addition to the usual constituents of the quartz based minerals, such as Aluminium, Al, silicon Si and oxygen O. 
The other related crystals from this family are the attractive labradorite, moonstone, sunstone, and amazonite. Orthoclase crystal, as its name already indicates, is a member of the orthoclase feldspars, The most important feature of this class is that they contain K and Na, in contrast to Ca and Na contained in the plagioclase.
A typical orthoclase is a solid solution of KAlSi3O8 and albite NaAlSi3O8. The origin of its name is probably from the Greek name for e or transparent. The golden variation can be found on Madagascar and in Mexico. The other colours are more frequent. If of gemstone quality orthoclase resembles golden or yellow beryl, also known as heliodor
Orthoclase an important mineral for industrial applications. Amongst the other uses, it is a source material for porcelain production. Porcelain (the vitrified ceramic of white colour) is produced from a mixture of orthoclase, quartz and kaolin.

History and Introduction

Orthoclase is a rare gemstone that belongs to the feldspar group, which also includes moonstone, amazonite, spectrolite and labradorite. Therefore, orthoclase is sometimes also called "K-spar". Orthoclase gemstones are typically champagne yellow, golden yellow or greenish, though they can be other colours. The name, "orthoclase" is a Greek word meaning "straight fracture", which refers to the cleavage planes of orthoclase being at right angles to each other. Orthoclase is faceted into attractive gemstones and also has an industrial use as a scouring agent in the making of some glasses and ceramics.

Identifying Orthoclase

Orthoclase is difficult to identify from other gemstones by appearance alone. It can be distinguished from similar gemstones by its hardness, low birefringence colours and perfect cleavage.

Orthoclase: Origin and Gemstone Sources

Orthoclase is found in Madagascar, Burma (Myanmar) and Kenya.

Determining Orthoclase Gemstone Value

Orthoclase Colour

Orthoclase is often colourless or champagne-coloured, but it can also be pink, green, orange or bluish. Gem quality materials are typically champagne, golden yellow or greenish.

Orthoclase Clarity and Lustre

Orthoclase is opaque to transparent and good quality specimens are eye clean. It has a vitreous lustre. Materials that display opalescence are classed as moonstone.

Orthoclase Cut and Shape

Transparent materials are faceted and opaque materials are usually cut en cabochon.

Orthoclase Treatment

Orthoclase is not currently known to be treated or enhanced in any way.

Orthoclase Healing properties

Orthoclase is a highly beneficial crystal for reversing the declining processes in our body. Not only that, it will help you prevent the fear of getting old and make you understand that the Spirit is always staying young no matter in what condition your physical body is. 
This crystal protects the internal organs, strengthens the bones, boosts our energy, and restores our will, therefore effectively buying us some time for a quality and happy old-age life.

Orthoclase metaphysical properties

In the new age community, the pleasant and refined energies of this metaphysical semi precious crystal are associated with hope, new beginnings, better perception, and discernment. It can be useful for revealing emotional tensions. It can be used for chakra balancing and meridian activation. This stone of cooperation and group work, encourages finding novel and unconventional paths toward achieving our goals. 
Besides being useful in joint ventures, this gemstone encourages inner growth and conducting constructive actions and carefully planned agendas. Primarily associated with the solar plexus chakra, it can enhance our will and knowledge as to how to improve our relationship to the Universe and Its powers.

Properties of Orthoclase

Chemical FormulaKAlSi3O8
ColourGreen, Yellow
Hardness6 - 6.5
Crystal SystemMonoclinic
Refractive Index1.52 - 1.53
SG2.56 - 2.59
TransparencyTransparent
Double Refraction-.008
LusterVitreous
Cleavage2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal
Mineral ClassSanidine

Sunday, 8 January 2017

Different shale distributions in low resistivity log response.


                   First, we will start with a small introduction about the resistivity logs                                           

Resistivity log

Technique : produce a current in the adjacent formation and measure the response of the formation to that current.

Resistivity logs are used to:

• determine hydrocarbon-bearing versus water bearing zones
• indicate permeable zones
• determine porosity

By far the most important use of resistivity logs is the determination of hydrocarbon-bearing versus water-bearing zones. Because the rock’s matrix or grains are non-conductive and any hydrocarbons in the pores are also non-conductive, the ability of the rock to transmit a current is almost entirely a function of water in the pores. As the hydrocarbon saturation of the pores increases (as the water saturation decreases), the formation’s resistivity increases. As the salinity of the water in the pores decreases , the rock’s resistivity also increases.

Resistivity tools principle : there are two types of resistivity tool , The dual lateral log ( DLL ) and the induction log ( DIL ) both types measures the resistivity in three zones simultaneously.

LLD looks deep into reservoir

LLS Looks shallow into the reservoir


MSFL reads the resistivity close to the wellbore.




Low Resistivity response :

High deep resistivity means : HCs or Tight streak  { low porosity }

Low deep resistivity means : Shale or wet sand.



Shale: Shale is defined as a fine-grained, indurated detrital sedimentary rock formed by the consolidation (by compression or cementation) of clay, silt, or mud.
It is characterized by a finely stratified structure of laminae ranging from 0.1 to 0.4 mm thick. Shale contains an appreciable content of clay minerals or derivatives from clay minerals, with a high content of detrital quartz; containing at least 50% silt, with 35% clay or mica fraction, and 15% chemical or authigenic materials
In petrophysical analysis, shale volume is one of the key answers used later to correct porosity and water saturation for the effects of clay bound water, (CBW).


Shale distribution in shaly sand :
 
Shale can be distributed in several different ways, as shown below.

Laminated shale is a special case in petrophysical analysis. Standard models for porosity and saturation do not work.
Dispersed shale is usually composed of from clay minerals that form in place after deposition due to chemical reactions between the rock minerals and the chemicals in the formation water.
Structural shale is usually deposited as particles, grains, or clasts during the initial depositional phase. For example, the flooding of a river valley can carry mud or shale from surrounding areas.
Different shale distributions have different effect on the sand reservoir.
In a sand reservoir contain structure shale : it will affect the reservoir porosity
In a sand reservoir contain laminae shale : it will affect only the net pay of the reservoir

In a sand reservoir contain : it will affect the porosity and permeability of the reservoir and also it will lead to a shortcut in the resistivity log response , which may result in a miss lead in the interpretation of the reservoir porosity and saturation  , it could be interpreted as sand bearing water instead of a sand contain dispersed shale.
So, the question here is how to differentiate between them and to avoid this wrong interpretation ?!
Let’s assume that you have a 100% clean sand reservoir. So the total porosity of this reservoir is 30% and the sand grains will represent 70% of the volume of the reservoir
Hint : Porosity of sandstone is 30 % and porosity of shale is 10%

Case 1 :

In the case of the presence of structure shale ,
So in this case shale grains will replace sand grains ( volume of 70% ) , the shale will bring its 10% porosity with it.
In other words , The porosity will be enhanced by 10% in the volume of 70% of the sand
So , the porosity will increase by 70/10 and the total porosity will be = 37 %


Case 2 :
In the case of the presence of laminae shale , in this case shale will replace the whole reservoir ( 100 & ) and also will bring its own 10% porosity.
In other words , the porosity will be reduced from 30% to 10%
Case 3 :
In the case of the presence of dispersed shale , in this case shale we will replace the porosity  volume it self ( 30 % ) and as usual it will bring its own porosity.
 
In other words , the porosity will be reduced into 3% ( 30 / 10 )Summarized figure for the different shale distributions in shaly sand reservoir and it’s effect on the reservoir porosity.
Shale distribution model proposed by Thomas and Stieber (Tyagi et al. 2009). Here Vshale is the volume of shale, φtotal is the total porosity, φmax is the maximum porosity, and φsh is the porosity in shale

Conclusion :


 So, we can differentiate between the three different types of shale distribution and according to the type we can make the right interpretation for the porosity and the saturation of the sand reservoir , also we will avoid the miss leading interpretation in the shortcut in the resistivity log.


Photo Credits: Ahmed Adel
Originally blog is written by Ahmed Adel 










Siccar Point - the world's most important geological site and the birthplace of modern geology


Siccar Point
 is world-famous as the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.

James Hutton unconformity with annotations - Siccar Point 


In 1788, James Hutton first discovered Siccar Point, and understood its significance. It is by far the most spectacular of several unconformities that he discovered in Scotland, and very important in helping Hutton to explain his ideas about the processes of the Earth.
At Siccar Point, gently sloping strata of 370-million-year-old Famennian Late Devonian Old Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.
Standing on the angular unconformity at Siccar Point (click to enlarge). Photo: Chris Rowan, 2009
As above, with annotations. Photo: Chris Rowan, 2009





Hutton used Siccar Point to demonstrate the cycle of deposition, folding, erosion and further deposition that the unconformity represents. He understood the implication of unconformities in the evidence that they provided for the enormity of geological time and the antiquity of planet Earth, in contrast to the biblical teaching of the creation of the Earth. 

   
How the unconformity at Siccar Point formed.


At this range, it is easy to spot that the contact between the two units is sharp, but it is not completely flat. Furthermore, the lowest part of the overlying Old Red Sandstone contains fragments of rock that are considerably larger than sand; some are at least as large as your fist, and many of the fragments in this basal conglomerate are bits of the underlying Silurian greywacke. These are all signs that the greywackes were exposed at the surface, being eroded, for a considerable period of time before the Old Red Sandstone was laid down on top of them.
The irregular topography and basal conglomerate show that this is an erosional contact. Photo: Chris Rowan, 2009

The Siccar Point which is a rocky promontory in the county of Berwickshire on the east coast of Scotland.

Credits: Chris Rowan




Saturday, 7 January 2017

Opal

What is Opal?

Opal is a gem quality and one of the most spectacular gemstones. A single stone can flash every colour of the spectrum with an intensity and quality of colour that can surpass the "fire" of diamond. The best opals can command prices per carat that rival the most expensive diamonds, rubies and emeralds. They are very popular gems. 
Opal is a wonderful stone for earrings, pendants, brooches and rings. However, it is softer than most other gemstones. Opal has a hardness of about 5.5 to 6.0 on the Mohs hardness scale. Because of that opal works best in earrings, brooches and other pieces that rarely encounter skuffs and impacts. When used in a ring the best designs have a bezel that protects the stone - instead of being placed in a prong setting that allows the edges of the stone to be exposed. 

History and Introduction

Opal is a gem-quality form of hydrated amorphous silicon dioxide. Its name is derived from the Sanskrit word for 'stone'. It is gemologically classed as a mineraloid rather than a mineral, owing to its amorphous form. Opal is considered to be the national gemstone for Australia, owing to the fact that Australia produces roughly 97% of the world's entire supply of opal gemstones. Opals can be divided into three main subgroups: precious opal, fire opal and common opal (potch).
Opal is famed for its ability to diffract light. The exact cause of opal's unique properties was only recently discovered by Australian scientists in the 1960s after analysis with electron microscopes. It was discovered that small spheres of silica gel caused interference, refraction and diffraction of light, resulting in opal's distinctive play of colour. The varying refractive indices of the spheres and spaces between them dissect the light on its passage through the stone. As light enters the opal, it bends around the tiny particles or 'spheres' of hydrated silica, as well as 'chips' of silicon and oxygen suspended within the stone. Light is comprised of all visible colours and can produce an entire spectrum of colours when it is diffracted.
Precious opal is known for its remarkable ability to diffract light, which results in rainbow-like colours that change with the angle of observation - known as 'play of colour'. Fire opal can sometimes exhibit slight colour play, but it is better known for its vivid body colour. Common opal is usually opaque, rarely translucent, and lacks play of colour. It is often found mixed with other gemstones, such as agate opal or moss opal. Common opal is known to exhibit 'opalescence'. The term 'opalescence' is often mistaken for 'play of colour'. Opalescence should technically only be used to describe the optical effects seen in common opal. Opalescence is caused by the reflection of light and appears as a sheen of light, typically milky-bluish in colour. It is a form of adularescence, whereas 'play of colour' is iridescence caused by light diffraction.

Sources of Opal

Although opal is found throughout the world, almost all of that opal is common opal of very little value. Most of the precious opal deposits that have been discovered are in Australia. The mines of Australia produce at least 90% of the world's precious opal. 
Famous mining areas in Australia include: Coober Pedy, Mintabie, Andamooka, Lightning Ridge, Yowah, Koroit, Jundah and Quilpie. Other countries that produce precious and fancy varieties of common opal include: United States, Mexico, Hungary, Indonesia, Brazil, Peru, Honduras, Guatemala, Nicaragua, Slovakia, Czech Republic and Ethiopia.

Identifying Opal

Opal by definition is hydrated silicon dioxide, always containing from three to thirty percent water. This characteristic and its relative softness with a rating of 5.5 to 6.5 makes opal quite easy to identify and distinguish from other gemstones. Unlike other gems with play of colour or iridescent effects, opal also exhibits 'opalescence'. Other iridescent stones do not display this pearl-like bluish colour effect that appears to glide across opal, but instead, will usually show colour that will abruptly appear, disappear and then reappear, depending on the viewing angle. In most cases, close observance of colour and optical phenomena can usually help distinguish opal from other similar gemstones.

Wonderful Names Used to Describe Opal

There are many types of opal and a wide variety of names are used to communicate about them. If you have spent a small amount of time looking at opal you have probably been surprised by this extensive vocabulary of wonderful names. There is actually a logic behind names such as fire opal, black opal, jelly opal, boulder opal, matrix opal, Coober Pedy, Mintabie, Andamooka, precious opal, opal doublet, and opal triplet. This post will describe the logic behind the names, see below.

Basic types of Opal: Precious Opal - Common Opal - Fire Opal

Precious Opal

"Precious opal" flashes iridescent colours when it is viewed from different angles, when the stone is moved or when the light source is moved. This phenomenon is known as a "play-of-colour". Precious opal can flash a number of colours such as bright yellow, orange, green, blue, red or purple. Play-of-colour is what makes opal a popular gem. The desirability of precious opal is based upon colour intensity, diversity, uniformity, pattern and ability to be seen from any angle. 
Precious opal is very rare and found in a limited number of locations worldwide. Most precious opal has been mined in Australia, secondary sources include: Mexico, Brazil, and the United States. Canada, Honduras, Indonesia, Zambia, Guatemala, Poland, Peru, New Zealand and Ethiopia. 

Common Opal

"Common opal" does not exhibit a "play-of-colour". It is given the name "common" because it is found in many locations throughout the world. Most specimens of common opal are also "common" in appearance and do not attract commercial attention. 
However, some specimens of common opal are attractive, colourful and lustrous. They can be cut into gemstones that accept a high polish. They can be beautiful but simply lack a play-of-colour that would earn them the name "precious". Common opal is frequently cut as a gemstone and can command reasonable prices. 

Fire Opal

"Fire Opal" is a term used for colourful, transparent to translucent opal that has a bright fire-like background colour of yellow, orange or red. It may or may not exhibit a "play-of-colour". The colour of fire opal can be as vivid as seen in the three stones shown in the photograph at right. 
Some people are confused by the term "fire opal". When they hear the word "fire" they immediately think of the flashes of spectral colour, known as "fire" that are produced by gem-quality diamond. Or, they think of the flashes of spectral colour, known as "play-of-colour" that is produced by precious opal. Fire opal might exhibit flashes of colour but such a display is usually weak or absent. Fire opal is simply a specimen of opal with a wonderful fire-like background colour. The colour is what defines the stone.

Precious Fire Opal

If you understand the difference between “precious opal” and “fire opal,” here is another variation. This opal from Ethiopia has an orange body colour, making it a “fire opal,” and it also contains an electric green to purple play-of-colour, making it a “precious opal.” So, we might call this a “precious fire opal.” Much of the Ethiopian opal currently being produced has yellow, orange or reddish body colour, along with play-of-colour, that allows it to be called “precious fire opal.”

Opal Names: Based Upon Opal and Host Rock Relationships

Solid Opal (Type 1 Opal)

"Solid opal" is a name used for a rough or cut stone that consists entirely of opal material without any host rock or other significant inclusions contained within the stone. Solid opal can be a combination of precious opal and common opal. Solid opal is also known as "Type 1 Opal".

Boulder Opal (Type 2 Opal)

"Boulder opal" is a term used for a rough or a cut gemstone that displays opal within its host rock. Opal often forms within voids or fractures in its host rock and specimens of boulder opal reveal this aspect of opal's origin. The contrast of colour can be striking when a bright flash of opal is seen within a the surrounding rock material. Many people enjoy the natural appearance of boulder opal and find these gemstones to be beautiful, interesting and educational. Boulder opal is also known as "Type 2 Opal".

Matrix Opal (Type 3 Opal)

"Matrix opal" is a term used for rough or finished gemstones in which precious opal is in an intimate mixture with the parent rock instead of the opal being confined to seams and patches as in boulder opal. Its material is often known as "Honduras Black Opal" because of its black base colour and pinfire appearance. Matrix opal is also known as "Type 3 Opal".

Opal Names Determined by Base Colour

White Opal or Light Opal

"Light opal" and "white opal" are terms used for opal material that has a white, yellow or cream body colour. This is the most common body colour for precious opal.

Black Opal or Dark Opal

"Black opal" is a term used for opal that has a dark body colour, often black or dark gray. The term is also used for opal that has a dark blue or dark green body colour. The dark body colour often makes the fire of black opal more obvious. This contrast of fire colour to body colour makes black opals very desirable and sold for high prices.

Crystal Opal

"Crystal opal" is a term used for a transparent to translucent opal material that has a play-of-color within the stone.

Blue Opal

"Blue Opal" are mined in the country of Peru in South America which is called "Peruvian blue opal". Although this stone is common opal that does not have a play of colour it is nevertheless very desirable because of its beautiful blue body colour.

Pink Opal

Pink opal occurs in shades of pink. The pink opal beads were made from common opal mined in Peru.

Morado Opal

"Morado" is the Spanish word for "purple". Some common opal with a purple base colour produced in Mexico has been given the name "Morado Opal".

Names That Describe an Opal's Fire Pattern

Harlequin Opal

"Harlequin opal" is a name given to an opal with patches of colour in the shape of rectangles or diamonds.

Contra-Luz Color Play

"Contra-Luz" is a name used for a colour-play that is visible when the light source is behind the stone. This effect only occurs in stones that are transparent or nearly transparent.

Pinfire Opal

"Pinfire opal" is a name used for opal that has pinpoints of fire throughout the stone.

Cat's-eye Opal

Rarely, opal will have fire that yields an optical effect similar to a cat's-eye. In these opals a thin line of fire will be visible from multiple directions and track back and forth across the stone similar to the cat's-eye known in other stones.

Opal Names Determined by Geography

Andamooka Opal

Andamooka is one of the early mining districts of South Australia. Commercial production began there in the 1920's. The area is famous for its matrix opal.

Coober Pedy Opal

Coober Pedy is a small town in South Australia that was first settled in 1916 when mining for opals began. It was one of the early prolific producing areas and has earned the nickname of "Opal Capital of the World". Coober Pedy is famous for producing white base-colour opals and production has continued uninterrupted since 1916.

Ethiopian Opal

Gem-quality opal from Ethiopia began entering the market in significant amounts starting in 1994. Since then, additional opal deposits have been discovered that might be large enough in size to take significant market share away from Australia, which has supplied nearly 100% of the opal market for over 100 years. Precious opal, fire opal, and very attractive common opal are all being produced in Ethiopia. They are becoming more abundant in the gem and jewellery market and more popular with consumers.

Honduras Black Opal

Honduras is well known for producing a black opal with a matrix or pinfire distribution of fire. Most people who know opal will know exactly what you are talking about if you use the term "Honduras Black Opal".

Lightning Ridge Opal

Lightning Ridge is a town in New South Wales, Australia that has become world-famous for its deposits of black opal. More black opals have been produced at Lightning Ridge than at any other location in the world. 

Louisiana Opal

"Louisiana opal" is a quartzite cemented with precious opal that has been mined in Vernon Parish, Louisiana. On close examination you can clearly see quartz grains with the spaces between them filled with a matrix of clear cement that produces a play of colour in incident light. It is a stable material that can be cut into cabochons, spheres and other objects.

Peruvian Opal

"Blue Opal" are mined in the country of Peru in South America which is called "Peruvian blue opal".Also some pink opal beads made from common opal mined in Peru. Although these stones are common opal that does not have a play of colour they are nevertheless very desirable because of their beautiful blue body colour.

Names Used for Assembled Stones

Assembled Stones

Most cut opals are solid stones. The entire stone is cut from a single piece of rough (see top illustration). However, some opal rough has very thin but brilliant fire layers. Some artisans cut the stone down to the thin color layer and glue it to a base of obsidian, potch or basalt - then cut a finished stone. These two part stones are called "opal doublets" (see center illustration). To protect the soft opal from abrasion and impact a crystal clear top of quartz, spinel or other transparent material is sometimes glued onto the opal. This produces a three part stone, called an "opal triplet" (see in the bottom illustration the clear cap, opal layer and base).

Opal Doublet

This stone is an opal doublet that was assembled from a thin layer of precious opal glued to a backing of parent rock. If this stone was mounted in a setting with a cup bezel it might be impossible to tell if it was a solid opal or a doublet.

Opal Triplet

Opal triplets produced by sandwiching a thin layer of precious opal between a backing of obsidian and a cover made of clear spinel. The clear top acts like a magnifying lens and enhances the appearance of the thin precious layer. The black obsidian back provides a contrasting background that makes the play of colour in the precious layer more obvious. If you look very closely at the inverted stone you will see a tiny line of colour that is the edge of a thin slice of precious opal.

Names Used for Opal and Opal Look-Alike

Natural Opal

Because of opal's beauty and desirability people have been producing materials that look like opal for nearly a century. A person with a little experience can easily recognise most of the "look-alike". "Natural opal" is the name used for genuine opal that has been mined from the Earth. It is genuine opal made by nature and not by humans. 

Synthetic Opal

"Synthetic opals" or "lab-created opals" have been made by humans. To be called "synthetic opal" they must be made from a material that has the same chemical composition (hydrated silicon dioxide) as natural opal. Synthetic opals have been made since the 1930's. Some synthetics look very much like genuine opal. However, most are easily recognised by a fire pattern that looks like thin snippets of foil embedded in a glassy matrix or streaks and spots of fire with a geometric shape.

Trade Name Materials

Some synthetic and imitation opals have been sold under trade names that have been at least temporarily popular. A material sold as "Gilson Opal" or "Gilsonite" is a lab-created material with a chemical composition that departs from the hydrous silicon dioxide chemistry of natural opals. "Slocum Stone" or "Slocum Opal" is another variety of opal simulant. During the 1960s and 1970s these and other opal silmulants were popular. These materials can be beautiful with wonderful colour. A hint that you are looking at one of these early opal simulants is when you see play-of-colour that is in a patchy to blocky pattern. 

Imitation Opal

"Imitation opals" are made from plastic or another glassy substance that is not silicon dioxide. They usually have a pearly "opalescence" rather than a genuine "play-of-colour". Plastic and glassy materials are sometimes called "opalite" when sold in stores.

Opal Trivia

Fluorescent Opal

Most opal will glow or fluoresce weakly under an ultraviolet lamp. However, some speciments exhibit a spectacular fluorescence. This specimen of mossy common opal rough from Virgin Valley, Nevada fluoresces a brilliant green under UV light.

Opalite

Opalite is a name given to an impure variety of common opal that can contain plumes, moss or other inclusions. The name "opalite" can be confused with plastic or glassy materials - imitation opals - that are sold under the same name.

"Water Opal" or Hyalite

Some opal does not exhibt a "play of colour", does not have a base colour, and does not have a body colour like most common opals. But this material is still opal. The tumbled opals at right are examples of this material. It has been called "water opal" and "hyalite".

Opals on Mars?

In 2008, NASA's Mars Reconnaissance Orbiter has discovered a number of opal deposits on Mars. In the satellite image at right, the ground the surface in the pinkish cream-coloured area to the right of the impact crater is covered with hydrated silica rock debris that we would call "opal". Mars researchers have also identified layers of opal exposed in the outcrops of crater walls. Since opal is a hydrated silicate its formation requires water. So, the discovery of opal on Mars is another evidence that water once existed on the planet. Image by NASA.

Properties of Opal

Chemical FormulaSiO2 · nH2O
ColourWhite, Colorless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Banded, Multicoloured
Hardness5.5 - 6.5
Crystal SystemAmorphous
Refractive Index1.37 - 1.47
SG1.98 - 2.25
TransparencyTransparent to translucent
Double RefractionNone
LusterVitreous, pearly, waxy
CleavageNone
Mineral ClassOpal