Ruby and Sapphire Specific Gravity  The specific gravity of a gemstone is a measurement indicating how heavy(dense) that gemstone is, it shows the ratio of a weight of the gemstone compare to the weight of an equal volume of water. Specific Gravity The SG of pure corundum, synthetic colourless sapphire, is 3.989, but natural 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.