Diamonds are part of the Carbon family of minerals. That is, diamonds are made up almost entirely of carbon atoms and they are arranged in a special shape that makes diamond crystals very hard.
Impurities in tiny amounts give us coloured diamonds, some of which are quite common and can make the diamond less valuable. Other colours are incredibly rare and make the diamond extremely valuable.
Other members of the Carbon family include Graphite. Graphite crystals are arranged in a way that makes them very soft so that graphite can be used in pencils.