Physical Properties
Specific Gravity
The SG of pure corundum, synthetic colourless sapphire, is 3.989, but natu- ral ruby and sapphire usually have SG near 3.997: there is no significant variation for specimens from different localities though iron-rich yellow, green and blue-green sapphire from Australia or Thailand may reach 4.00.
Hardness, Fracture, Cleavage and Parting
Corundum is 9 on Mohs’ scale. Ruby and sapphire may be brittle and should be carefully handled. The fracture varies from uneven to conchoidal.
There is no cleavage. Parting, from exsolved bohmite, may occur parallel to the basal plane or rhombohedral faces.
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