Colour in diamonds is caused by the presence of minor elements other than pure carbon. A yellowish or brownish tinge often occurs and is regarded as an imperfection. Two other important characteristics of the diamond are its brilliancy and fire. The physical properties which determine luster and fire are higher for diamond than for any other natural, transparent, colourless stone. Uncut diamonds have a greasy luster and are not brilliant, but the same stones when cut exhibit a high luster, characterized technically as adamantine.
Diamonds are excellent conductors, and are cold to the touch.
Most diamonds are not good electrical conductors and become charged with positive electricity when rubbed. Another important physical characteristic of the diamond is its resistance to attack by acids or alkalies. Tremendous heat and pressure are required for the crystallization of carbon into diamond. Diamonds were produced in molten rock or magma, in which these conditions prevail, far below the surface of the earth, and were carried to the surface by the magma, which later hardened. In rare circumstances they are carried to the surface by kimberlite pipes.