Chrysoprase
Chrysoprase, chrysophrase or chrysoprasus is a gemstone variety of chalcedony that contains small quantities of nickel. Its color is normally apple-green, but varies from turquoise-like cyan to deep green. The darker varieties of chrysoprase are also referred to as prase.
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 color of chrysoprase, rather than any pattern of markings, that makes it desirable. The word chrysoprase comes from the Greek language|Greek] chrysos meaning 'gold' and prasinon, meaning 'green'.
Unlike emerald which owes its green color to the presence of chromium, the color 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. 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 to 7 on the Mohs hardness scale and a conchoidal fracture like flint.