Chrysoprase or chrysophrase is a gemstone variety of chalcedony (itself a fibrous form of quartz) that contains small quantities of nickel. Its colour is normally apple-green, but varies to deep green. It 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 which are basically transparent and formed from large easily recognised six-sided crystals.
Other members of the cryptocrystalline quartz family include agate, carnelian, and onyx. Unlike many non-transparent members of the quartz family, it is the colour of chrysoprase, rather than any pattern of markings, that makes it desirable.
The word chrysoprase comes from the Greek chrysos meaning 'gold' and prason, meaning 'leek'. Due to its comparative scarcity and pleasing green colour, chrysoprase is one of the most prized varieties of quartz. Higher quality specimens often rival fine jade, for which it is sometimes mistaken. Cut into cabochons (smooth domed gems with flat backs for use in jewellery), it can be as sought after as fine amethyst. Unlike emerald which owes its beautiful green colour to the presence of chromium, the colour of chrysoprase is due to trace amounts of nickel in the structure. The nickel reportedly occurs as platelets of the talc-like mineral willemseite. Chrysoprase results from the deep weathering or lateritisation 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 quartz, chrysoprase has a hardness of 6 - 7 on the Mohs hardness scale and a conchoidal fracture like flint. The best known sources of chrysoprase are Queensland, Western Australia, Germany, Poland, Russia, Arizona, California, and Brazil.