What is diamond?
Diamond is the most commercially important gem worldwide. Some call it the king of gemstones, and it has been used in jewellery since antiquity, first as uncut crystals and then after the 1300s as cut stones. Diamond is the hardest known naturally occurring material, with Mohs' hardness 10.
It is most familiar as a colourless or near colourless gem, but a whole spectrum of colours are also known, including yellow, pink, green, red and blue. These coloured diamonds can command the very highest prices and are thus the most valuable of gemstones. They are referred to as fancy-coloured diamonds.