A diamond (from the ancient Greek ἀδάμας, adámas) is a very highly prized gem. Its hardness and high dispersion of light, or "brightness", make it useful for industrial applications and jewellery. It is the hardest known mineral. It is the allotrope of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. After graphite, diamond is the second most stable form of carbon.