corundum
Mineral, Al2O3; sp. gr. 3.9–4.1; hardness 9; trigonal; the two main varieties blue and green, but can be yellow, or brown to almost black, and transparent; adamantine to vitreous lustre; crystals ...
gems
Minerals (usually of crystallized matter) used for decorating items such as textiles and liturgical objects or for personal adornment. The most highly prized are the precious stones, diamonds, ...
precious stone
One of the four intrinsically valuable gemstones, diamond, sapphire, ruby, and emerald, which are used in jewellery.
ruby
The transparent red variety of the mineral corundum, the colour being due to the presence of traces of chromium. It is a valuable gemstone, more precious than diamonds. The finest rubies are obtained ...
