The ruby belongs to the corundum family, which are crystals mainly composed of aluminium and oxygen. They are touted for their extreme hardness (9 on the mohs scale) and durability, making them very easy and safe to wear.
Rubies are very similar to sapphires. In fact, they are literally red sapphires. Their chemical structure differs in that rubies display a presence of chrome in the crystal.
They display a myriad of red hues, ranging from a warm red, to an opaque rose; and even a deep, crimson red.