Concentrations of diamonds great enough to be economically feasible for mining are usually found in Earth’s oldest continental regions, called cratons. Cratons form the cores of most continents and consist of inactive geological areas more than 2 billion years old with thick crust and deep roots extending into the mantle beneath. Craton conditions are ideal for diamond formation and preservation. Scientists have determined the ages of some diamonds by dating mineral impurities trapped within the diamonds. These data reveal that most cratonic diamonds are ancient, some older than 3 billion years.

Much younger volcanic rocks—kimberlites and lamproites—pass through the cratonic rocks in a liquid form called magma during their rapid ascent to Earth’s surface. These flowing veins of rock act as carriers of diamonds and other rock fragments. After eruption they solidify, forming funnel-shaped kimberlite “pipes.” These pipes are primary diamond deposits. Many diamonds are recovered at a distance from their primary deposits in secondary alluvial deposits, which are loose eroded materials left behind by flowing water. In some instances diamonds are also found in sandstones, conglomerates, and other sedimentary rocks that presumably solidified from former alluvial deposits. Wind and glaciers can also transport diamonds from their point of origin at Earth’s surface.

Small, generally low quality diamonds form in rocks at shallower depths under pressure conditions that are higher than usual for those depths. Tectonic movement, rather than magma, transports these diamonds to Earth’s surface. Deposits of this type occur in areas such as Kazakhstan and typically involve the collision of a continental and an oceanic plate followed by rapid uplift of deeply buried rocks. Diamond deposits brought to the surface by tectonic movement are generally younger than kimberlitic diamonds, and typically consist of microdiamonds (less than 1 mm across) or graphite relics of larger diamonds.

Diamonds are also found in meteorites and near meteorite craters on Earth’s surface. Extremely small diamonds (nanodiamonds) occur in many types of meteorites and have a lower density than other diamonds. Meteorites can also produce pressure and heat at the moment of impact sufficient to transform carbon into diamond. Diamond found in a type of meteorite called ureilite is thought to form directly from graphite contained in the meteorites upon impact. Impact-crater diamonds are opaque and range from very small to around a centimeter in diameter.

Rough diamonds are not brilliant and can appear greasy. Diamond cutting encompasses a number of processes that bring out the beauty of gem diamonds. These processes include cleaving, sawing or laser cutting, and polishing. A diamond cutter seeks to enhance the brilliance and fire of each stone and to eliminate imperfections, such as cracks and cloudiness. The cutter develops a plan that will accomplish these goals while still producing a gem of the greatest size and hence maximum value. About half of a natural diamond’s size is lost in diamond cutting.