The sapphire 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.
We mostly know sapphires for their blue color, which are due to the inclusion of titanium and iron traces. But they can also display a myriad of other blue hues, ranging from a foggy blue to a very dark, almost black color.
Sapphires also exist in many other varying colors : black, yellow, purple... The only exception is that a red sapphire is in fact a ruby.