With a hardness of 9 on the Mohs scale, corundum is one of the hardest minerals in the world. Only diamonds are harder. Corundum is colourless by nature, but can take on other colours in the presence of chrome or iron inclusions. The two best known gemstone variants are ruby (corundum with chromium inclusions) and sapphire (corundum with iron inclusions).
The red variety of corundum is known as ruby. The mineral is characterised by its colour and the formation of hexagonal crystals. Clear rubies are considered gemstones and are very valuable. Ruby is seen as a strong stimulating stone. It stimulates the blood circulation and therefore sensuality. This mineral has been known for centuries as the stone for passion, protection and prosperity.
The blue variant of corundum is known as sapphire. However, all corundum minerals that are not red are also classified as sapphire. There are also yellow, pink, violet and green sapphires. Sapphire helps you to think clearly and to be concentrated and focused.