Ruby gemstones. Rubies are a red variety of the mineral corundum (aluminium oxide). It is the second hardest mineral after diamond, rated 9 out of 10 on the Mohs scale. Artificial rubies are used in industrial lasers and also in analogue watch mechanisms. Inclusions of the mineral rutile (titanium oxide, not seen) cause a six- pointed-star light effect (asterism) to form star ruby.
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