Ruby is the red variety of the mineral corundum, one of the hardest minerals on Earth. 

Corundum is the mineral form of alumina, which crystallizes in the hexagonal system.

The red color of ruby results from a small admixture of chromic oxide.

Only red corundum is entitled to be called ruby, all other colors being classified as sapphires. ❤️💙

The most prized tint is blood red or crimson known in the trade as 'pigeon's blood' red. 🐦🟥