Diamond

Diamond is a native crystalline form of carbon in which every atom is covalently bonded to four neighbours in a rigid cubic lattice. That structure makes it the hardest known natural material, a 10 on the Mohs scale, and gives it exceptional resistance to scratching. Its high refractive index (about 2.42) and strong dispersion produce the fire and brilliance prized in cut stones.

Most diamonds crystallised in the Earth's mantle over a billion years ago and were carried upward by deep volcanic kimberlite and lamproite pipes. Colourless stones are graded on the classic "four Cs" — carat, colour, clarity and cut — while trace elements produce fancy colours: nitrogen yields yellow, boron blue. Major sources include Botswana, Russia, Canada and, historically, South Africa and India.