Ruby is an instance of the mineral Corundum. In fact the only difference between a Sapphire and a Ruby is the color. A Corundum of any color other than red is a Sapphire. Because it is extremeley hard and durable it is well suited for jewelry. Highest quality Rubies are frequently valued above even Diamonds. Rubies over two carats are extremely rare and therefore valuable.
Generally, Rubies with real red color are more valuable than those that are darker due to purple hues.
Rubies rate 9 on the hardness scale and only Diamond beats them among gemstones.