Chrome chalcedony

Chrome Chalcedony
Zimbabwean chrome chalcedony (known locally as mtorolite). Scale is in centimetres.
General
Category	Oxide mineral
Chemical formula	Silica (SiO2)
Identification
Molar mass	60 g / mol
Color	Emerald green
Crystal system	Trigonal and Monoclinic [1]
Cleavage	Absent [2]
Fracture	Uneven, splintery, conchoidal [2]
Mohs scale hardness	6 - 7 [2]
Luster	Vitreous, dull, greasy, silky [2]
Streak	White
Diaphaneity	Translucent [2]
Specific gravity	Usually 2.60. Sometimes greater than 2.62 [3]

Chrome chalcedony is an green variety of the mineral chalcedony, colored by small quantities of chromium.^[4] It is most commonly found in Zimbabwe, where it is known as Mtorolite,^[5] Mtorodite,^[6] or Matorolite.^[7]

Chemical composition

Chrome chalcedony is similar in appearance to the better known chrysoprase, but differs in that whilst chrome chalcedony is colored by chromium (as chromium(III) oxide), chrysoprase is colored by nickel.^[4] The two can be distinguished with a Chelsea color filter, as chrome chalcedony will appear red, whilst chrysoprase will appear green.^[4][8] Chrome chalcedony (unlike chrysoprase) may also contain tiny black specks of chromite.^[3]

Chrome chalcedony is (together with agate, carnelian, chrysoprase, heliotrope, onyx and others) a variety of chalcedony. This is a cryptocrystalline form of silica, consisting of fine intergrowths of the minerals quartz and moganite.^[1]

Locality

Chrome chalcedony (known as mtorolite, mtorodite or matorolite) occurs in Zimbabwe, principally near to the mining town of Mtoroshanga, located on the Great Dyke geological feature.^[5] It has also been discovered in western Australia, the Balkans, Bolivia, Turkey and the Ural mountains.^[9]

Ancient history

Chrome chalcedony was widely used in jewellery and seals throughout the Roman Empire. The source of the mineral is unclear, as whilst Pliny the Elder described it as coming from India, no deposits have been found there. It may have come from Anatolia (in modern day Turkey) where deposits are known to exist.^[9]

Chrome chalcedony disappeared from use sometime in the 2nd century. It was only rediscovered when the Zimbabwean deposits were found in the 1950s.^[10]

References

1. ^ ^{a b} Heaney, Peter J. (1994). "Structure and Chemistry of the low-pressure silica polymorphs". Reviews in Mineralogy and Geochemistry 29: 1–40. 
2. ^ ^{a b c d e} Duda, Rudolf; Rejl, Lubos (1990). Minerals of the World. Arch Cape Press. ISBN 0-517-68030-0 (Reference gives data for chalcedony in general.) 
3. ^ ^{a b} "Insider Gemologist: What Are the Identifying Characteristics of the Different Varieties of Green Chalcedony?". GIA Insider (Gemological Institute of America) 7 (13). 2005. http://www.gia.edu/newsroom/issue/2798/2372/insider_newsletter_details.cfm#3. 
4. ^ ^{a b c} Willing, M. J.; Stocklmayer, S.M (2003). "A new chrome chalcedony occurrence from Western Australia". Gems & Gemology 23: 265–279. 
5. ^ ^{a b} Cairncross, Bruce (2005). Field Guide to Rocks & Minerals of Southern Africa. Struik. ISBN 1868729850. 
6. ^ Hey, M. H.; Embrey, P. G. (1974). "Twenty-eighth list of new mineral names". Mineralogical Magazine 39: 903–932. doi:10.1180/minmag.1974.039.308.15. http://www.minersoc.org/pages/Archive-MM/Volume_39/39-308-903.pdf. ^{[dead link]}
7. ^ Hey, M. H. (1970). "Twenty-sixth list of new mineral names". Mineralogical Magazine 37: 954–967. doi:10.1180/minmag.1970.037.292.19. http://www.minersoc.org/pages/Archive-MM/Volume_37/37-292-954.pdf. ^{[dead link]}
8. ^ O'Donoghue, Michael (2006). Gems. Butterworth-Heinemann. ISBN 0750658568. 
9. ^ ^{a b} Lule-Whipp, Cigdem (2006). "Chromium Chalcedony from Turkey and Its Possible Archeological Connections". Gems & Gemology: Proceedings of the 4th International Gemological Symposium & GIA Gemological Research Conference. 42. pp. 115. http://lgdl.gia.edu/pdfs/grc_2006.pdf. 
10. ^ Thoresen, Lisbet. "Ancient Glyptic Art – Gem Engraving and Gem Carving". http://ancient-gems.lthoresen.com/. Retrieved 2009-01-13.