| Chrysoprase | |
|---|---|
 Chrysoprase (Poland) | |
| General | |
| Category | Tectosilicate minerals |
| Group | Quartz group |
| Variety of | Chalcedony |
| Formula | SiO2 |
| Crystal system | Trigonal (quartz),monoclinic (moganite) |
| Identification | |
| Color | Olive to applegreen |
| Crystal habit | Microcrystalline aggregates |
| Cleavage | None |
| Fracture | Conchoidal, granular |
| Tenacity | Brittle |
| Mohs scale hardness | 6.0–7.0 |
| Luster | Greasy, waxy |
| Diaphaneity | Nearly opaque to nearly transparent |
| Specific gravity | 2.651–2.91 |
| Birefringence | 0.004–0.009 |
| Dispersion | None |
| Ultravioletfluorescence | None |
| References | [1] |
Chrysoprase,chrysophrase orchrysoprasus is agemstone variety ofchalcedony (acryptocrystalline form ofsilica) that contains small quantities ofnickel. Its color is normally apple-green, but varies fromturquoise-like cyan to deep green. The darker varieties of chrysoprase are also referred to asprase. (However, the term prase is also used to describechlorite-included quartz, and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.)
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 includeagate,carnelian, andonyx. 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 theGreekχρυσόςchrysos meaning 'gold' andπράσινονprasinon, meaning 'green'.
Unlikeemerald which owes its green color to the presence ofchromium, the color of chrysoprase is due to trace amounts ofnickel compounds in the form of very small inclusions. The nickel reportedly occurs as different silicates, likekerolite orpimelite (not NiO mineral,bunsenite, as was reported before). Chrysoprase results from the deep weathering orlateritization of nickeliferousserpentinites or otherultramaficophiolite rocks. In the Australian deposits, chrysoprase occurs as veins and nodules with browngoethite and otheriron oxides in themagnesite-richsaprolite below an iron and silica cap.
As with all forms ofchalcedony, chrysoprase has a hardness of 6 to 7 on theMohs hardness scale and a conchoidal fracture likeflint.
The best known sources of chrysoprase areIndonesia,Queensland,Western Australia, HanetiTanzania,Germany,Poland,Russia,Arizona,California, andBrazil. Deposits in centralTanzania have been in constant production since 1986. The chrysoprase and nickel silicate ore deposit inSzklary,Lower Silesia,Poland, was probably the biggest European chrysoprase occurrence and possibly also the biggest in the world.[citation needed]
Chrysoprase was fashionable during the 1980s.[2]
A very similar mineral to chrysoprase ischrome chalcedony, in which the color is provided bychromium rather than nickel.
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| Cryptocrystalline | |||||||||
| Amorphous | |||||||||
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Gemmological classifications by E. Ya. Kievlenko (1980), updated | |||||||||
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