The September birthstone sapphire is one of the rarest gemstones on the planet. They are formed of the mineral corundum, the same mineral that July’s birthstone rubies originate from. The obvious difference being that rubies are the red form, and all other variations of corundum come under sapphires. Corundum is a very hard, tough and stable mineral it is the second hardest mineral after diamond, unaffected by acids and most environments. This makes sapphires a popular gemstone and the most stable to use in jewellery.