Diamonds form in rocks deep inside the Earth. The hot rock rises to near the Earth's surface and cools down. As the rock cools down the diamonds form. They form crystals which may be in the shape of a cube or an octahedron. The crystals may be transparent and without colour or they may be red, yellow, brown or even black. Crystals which are clear are used as gemstones to make jewellery.

Some diamonds are opaque and grey or black. They are called bort and are used for drilling, cutting and grinding. Diamond drills are used to cut through rock at an oil well. Diamonds are used for cutting and grinding other hard substances in factories.

Diamond is made from a substance called carbon. This is the same substance that makes graphite in a pencil and charcoal on a barbecue. It has been discovered that diamonds can also be made by heating and pressing carbon very hard. These artificial diamonds are called industrial diamonds but they are not as attractive as the diamonds that are made naturally and they are very small.