Diamond is the hardest naturally occurring mineral and one of the most precious gemstones. Hardness means that it can scratch any other substance but that it cannot be scratched. Diamonds are made of pure carbon just like the mineral graphite, but the atoms are packed more tightly together under mantle pressures. Diamond is associated with plutonic-volcanic formations, particularly the famous kimberlites, or “diamond pipes,” of South Africa. Diamonds are believed to have been found first in Borneo and India. Brazil was a major producer of diamonds in the 18th and 19th centuries. Diamonds were discovered in South Africa in 1867 on the shore of the Orange River near Hope Town, and several years later, primary deposits of diamonds were found on the plateau between the Modder and Vaal Rivers. These primary deposits were located in volcanic pipes containing a variety of peridotite (kimberlite). These pipes intersected the surface in circular or elliptical areas. Some diamonds were recovered on the surface from a weathered material called yellow ground, but others had to be extracted from the harder “blue ground” below. Important South African diamond mines include Bultfontein, De Beers, Du Toitspan, Kimberly, and Wesselton. For a time most of the world’s annual output of natural diamonds came from the pipe mines, but, later, alluvial deposits in Africa surpassed the pipe mines in production. Diamond production is not confined to Africa; diamonds have been recovered in parts of the world as widely separated as Mexico, Siberia, and Canada. Diamonds not used as gemstones have widespread industrial uses. Industrial- quality diamonds occur naturally in forms including ballas (spherical aggregates of diamond crystals), bort (crystals with irregular shape, bad flaws, and numerous inclusions; used in drilling and abrasives), and carbonado (a dark variety of diamond). Although a colorless diamond is pure carbon (as demonstrated by the fact that burning such a diamond in an oxygen atmosphere yields only carbon dioxide), other elements may occur in diamonds and affect the color of the stones; blue, green, and yellow gem-quality diamonds have been found.