Sapphire is defined as any instance of the mineral Corundum that is not red. In the case of a red version of the mineral it is considered a Ruby. Because it is extremeley hard and durable it is well suited for jewelry. The largest Sapphire ever presented at auction was a cushion-shaped sapphire weighing 478.68 carats and is associated with Queen Marie of Romania.
The name sapphire is derived from the Latin word for blue, "sapphirus". Sapphires are typically associated with the color blue, but they actually come in all colors except red
(a red Corundum is categorized as a Ruby).
They rate 9 on the hardness scale and only Diamond beats them among gemstones.