What is Chrysoprase?
Chrysoprase, chrysophrase or chrysoprasus is a gemstone variety of chalcedony (a cryptocrystalline form of silica) that contains small quantities of nickel. Its colour is normally apple-green, but varies to deep green. The darker varieties of chrysoprase are also referred to as prase. (However, the term prase is also used to describe chlorite-included quartz, and to a certain extent is a colour-descriptor, rather than a rigorously defined mineral variety.)
Chrysoprase is the green variety of Chalcedony. Its colour ranges from a light, minty-green to a deep apple-green. The unique, rich colour of Chrysoprase is caused by impurities of nickel, as this gemstone most frequently originates in nickel-rich Serpentine deposits.
Chrysoprase is cryptocrystalline, which means that it is composed of crystals so fine that they cannot be seen as distinct particles under normal magnification. This sets it apart from rock crystal, amethyst, citrine, and the other varieties of crystalline quartz. Other members of the cryptocrystalline silica family include agate, carnelian, and onyx. Unlike many non-transparent silica minerals, it is the colour of chrysoprase, rather than any pattern of markings, that makes it desirable. The word chrysoprase comes from the Greek χρυσός chrysos meaning 'gold' and πράσινον prasinon, meaning 'green'.
Unlike emerald which owes its green colour to the presence of chromium, the colour of chrysoprase is due to trace amounts of nickel compounds in the form of very small inclusions. The nickel reportedly occurs as different silicates, like kerolite or pimelite (not NiO mineral, bunsenite, as was reported before). Chrysoprase results from the deep weathering or lateritization of nickeliferous serpentinites or other ultramafic ophiolite rocks. In the Australian deposits, chrysoprase occurs as veins and nodules with brown goethite and other iron oxides in the magnesite-rich saprolite below an iron and silica cap.
As with all forms of chalcedony, chrysoprase has a hardness of 6–7 on the Mohs hardness scale and a conchoidal fracture like flint.
Occurrence
The best known sources of chrysoprase are Queensland, Western Australia, Germany, Poland, Russia, Arizona, California, and Brazil. The chrysoprase and Ni silicate ore deposit in Szklary, Lower Silesia, Poland, was probably the biggest European chrysoprase occurrence and possibly also the biggest in the world. A very similar mineral to chrysoprase is chrome chalcedony, in which the colour is provided by chromium rather than nickel.
Properties of Chrysoprase
| Chemical Formula | SiO2 (with a high level of nickel impurities) |
| Colour | Green |
| Hardness | 7 |
| Crystal System | Hexagonal |
| Refractive Index | 1.54 - 1.55 |
| SG | 2.63 - 2.65 |
| Transparency | Translucent to nearly opaque |
| Double Refraction | .009 |
| Luster | Vitreous to waxy |
| Cleavage | None |
| Mineral Class | Quartz (Chalcedony) |
