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