Diamonds are made of carbon atoms that are bonded together in a unique way to create crystal formations. Carbon is the same element that makes up coal or graphite used for pencils, but the placement of the atoms in diamonds is what makes them transparent and hard while coal and graphite are opaque and soft. Each carbon atom in a diamond is strongly bonded to four others, making them the hardest known natural substance. Diamonds were formed between one to three billion years ago, deep below the earths surface under intense heat and high pressure. Most diamonds were formed in the upper mantle of the earths crust, around a hundred miles beneath the surface. Diamonds were brought to the surface by volcanic eruptions that carried the already-formed diamonds from the upper mantle to the surface of the Earth. Once the diamonds are brought to the surface and cooled relatively quickly, those carbon atoms are locked into place and theres just not enough energy to now start rearranging them into graphite.