chalcedony

What is chalcedony?

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).
Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colours, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black. The colour of chalcedony sold commercially is often enhanced by dyeing or heating.
The name chalcedony comes from the Latin chalcedonius (alternatively spelled calchedonius). The name appears in Pliny the Elder's Naturalis Historia as a term for a translucid kind of Jaspis. The name is probably derived from the town Chalcedon in Asia Minor.. It is a hapax legomenon (the term for a word found nowhere else) so it is hard to tell whether the precious gem mentioned in the Bible is the same mineral known by this name today.

Varieties

Agate, Aventurine, Carnelian, Chrysoprase, Heliotrope, Moss agate, Mtorolite and Onyx.

History

As early as the Bronze Age chalcedony was in use in the Mediterranean region; for example, on Minoan Crete at the Palace of Knossos, chalcedony seals have been recovered dating to circa 1800 BC. People living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios, ring bezels (the upper faceted portion of a gem projecting from the ring setting), and beads that show strong Greco-Roman influence.
Fine examples of first century objects made from chalcedony, possibly Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan. Hot wax would not stick to it so it was often used to make seal impressions. The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadıköy district of Istanbul.
According to tradition, at least three varieties of chalcedony were used in the Jewish High Priest's Breastplate. (Jewish tradition states that Moses' brother Aaron wore the Breastplate, with inscribed gems representing the twelve tribes of Israel). The Breastplate supposedly included jasper, chrysoprase and sardonyx, and there is some debate as to whether other agates were also used.
In the 19th century Idar-Oberstein, Germany became the world's largest chalcedony processing centre, in particular agates. Most of these agates were from Latin America, in particular Brazil. Originally the agate carving industry around Idar and Oberstein was driven by local deposits that were mined in the 15th century. Several factors contributed to the re-emergence of Idar-Oberstein as agate centre of the world: ships brought agate nodules back as ballast, thus providing extremely cheap transport. Also, cheap labour and a superior knowledge of chemistry allowing them to dye the agates in any colour with processes that were kept secret helped. Each mill in Idar Oberstein had four or five grindstones. These were of red sandstone, obtained from Zweibrücken; and two men ordinarily worked together at the same stone.

Geochemistry

Structure

Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz. More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite. The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%. The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.

Solubility

Chalcedony is more soluble than quartz under low-temperature conditions, despite the two minerals being chemically identical. This is thought to be because chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio. It has also been suggested that the higher solubility is due to the moganite component.

Solubility of quartz and chalcedony in pure water

This table gives equilibrium concentrations of total dissolved silicon as calculated by PHREEQC (PH Redox Equilibrium (in C language, USGS)) using the llnl.dat database
TemperatureQuartz solubility (mg/L)Chalcedony solubility (mg/L)
0.01 °C0.681.34
25.0 °C2.644.92
50.0 °C6.9512.35
75.0 °C14.2124.23
100.0 °C24.5940.44

Healing properties of Chalcedony

Blue Chalcedony is an excellent crystal for public speakers and those who speak for a living. Lawyers and political speakers might benefit from touching this stone to the tip of their tongue while listening to their opponents to enhance their counter arguments.
Actors might rub it against their lips and throat, while singers may drink a glass of water in which Chalcedony has soaked for an hour before going on stage. Wearing this crystal around the neck helps overcome stage fright and the fear of public speaking. 
As a stone of peace and peace-making, Blue Chalcedony encourages stillness and calm in the home for those at odds with one another, vying for their place in the pecking order. It is also beneficial for daily journeys to work, or stressful trips involving children.

Physical Properties of Chalcedony

Chemical FormulaSiO2
ColourWhite, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Banded, Multicoloured
Hardness6.5 - 7
Crystal SystemHexagonal
Refractive Index1.54 - 1.55
SG2.63 - 2.65
TransparencyTranslucent to opaque
Double Refraction.009
LusterVitreous to waxy
CleavageNone
Mineral ClassQuartz (Chalcedony)