carbon
carbon
A nonmetallic element, found free in nature in three allotropic forms: amorphous, graphite, and diamond. A fourth form, known as "white" carbon, is now thought to exist. Symbol, C. Graphite is one of the softest known materials, while diamond is the hardest. Occurs as a constituent of coal, petroleum, natural gas, and all organic compounds. The isotope, carbon 14, is radioactive and is used as a tracer in biological and organic chemical research.
A gray-to-black, opaque, tough, hard cryptocrystalline aggregate of diamond crystals occurring in irregular shapes and sizes. It is classed as an industrial diamond and formerly was used extensively as a cutting-medium inset in diamond-drill bits. More recently, only occasionally used in diamond bits and other tools. Also called black diamond; carbonado.