"Rock of the Month # 14, posted for August 2002" a trio of colourful examples of corundum, Al₂O₃ --- a simple oxide responsible for two of the "big three" coloured gemstones (the third being emerald, a green beryl). Samples 1709 (ruby) and 1813 (sapphire), photo by Karyn Gorra.

The oxide corundum is widespread worldwide, but its gestation and preservation are sufficiently demanding that it is not a common mineral. It is generally quite pure, composed largely of aluminium and oxygen, two of the principal components of the Earth's crust. Impurities such as iron and chromium may impart characteristic colour, yet these seldom exceed 1 percent by weight. Corundum, in common with the garnet and feldspar families, leads a double life as both gemstone and industrial mineral. Extreme toughness and hardness (9 on the Moh's scale) ensured corundum a valuable place as an abrasive, although synthetic alternatives are now available. 100 years ago, however, corundum and garnet were both important industrial commodities, and the top producing region for abrasive corundum was the Grenville province of southeast Ontario (perhaps a future Rock of the Month).

Ruby is noted from parts of Thailand and Burma, India, Russia, Norway, Madagascar and elsewhere. Sapphire hails from Sri Lanka, Kashmir, eastern Australia, Montana and beyond. Hughes (1997) provides an encyclopaedic review of these captivating gemstones, while Karanth (2000) offers a perspective from the Indian subcontinent. The provenance of these gemstones, and the enhancements they may undergo in the gem trade, are of great practical interest, in such contexts as gemmology, archaeology, and even mineral exploration. Mary Garland (2002) provides the latest detailed investigation of corundum provenance, both worldwide and in the specific context of the placer deposits of western Montana. Her work explores the geology of the Montana gem localities and also the other avenues available to a sleuth of gemstone origins. These include the nature of mineral inclusions trapped within the corundum and the trace-element chemistry of the host mineral itself.

The striking hexagonal plate of ruby, 37x29x11 mm, is from India, probably the southern state of Karnataka (Mysore). It displays well-defined cleavage traces on the basal section. The two dull bluish striated hexagonal crystal sections of sapphire are each about 12 mm in diameter. They are from the state of Andhra Pradesh, eastern neighbour of Karnataka.

References

GARLAND,MI (2002) The Alluvial Sapphire Deposits of Western Montana. PhD Thesis, University of Toronto, 401pp.

HUGHES,RW (1997) Ruby & Sapphire. RWH Publishing, Boulder, CO, 512pp.

KARANTH,RV (2000) Gems and Gem Industry in India. Geol.Soc.India Memoir 45, 405pp. [especially pp.222-233]