When an element is found in several forms which have essentially different properties, it is said to be allotropic in character. Carbon is such an element, the different forms or modifications of which are the diamond, graphite, and pure amorphous carbon.

The diamond is pure, crystalline carbon. It has a specific gravity of 3.5 and is one of the hardest substances known. On account of its hardness it is used to cut glass. The black, impure variety, called carbonado, is set into the end of a drill, called a diamond drill, which is used for boring holes in hard substances.

Graphite is a soft, lead-colored, shiny solid often called "black lead" or plumbago because it was originally supposed to contain lead. It is smooth and greasy to the touch and is used in the form of flakes as a lubricant because it does not become decomposed, as do oils, by high temperatures and the heat of friction. Since graphite is soft, it readily wears away and when drawn across a piece of paper the friction causes it to pulverize and leave a mark on the paper. Hence its use in pencils. In addition, graphite serves as the basic substance in the making of stove polish and as an ingredient in the manufacture of certain crucibles in which metals are to be heated and melted.

Amorphous or non-crystalline carbon includes a number of varieties of coal, charcoal, lampblack, coke, and gas carbon.

Charcoal is a black, brittle solid and is obtained by heating wood in a closed pile without much access to air. The heat drives out the liquids and gases. These are collected as a by-product and distilled into wood alcohol, acetic acid, etc. Charcoal resists the action of moisture, heat, and air, and consequently telegraph and other poles are often charred before being put into the ground. It is also used as a disinfectant, because it absorbs gases. Gunpowder has a basis of charcoal. The charring of bones and animal refuse gives a form of charcoal called animal charcoal or bone-black, which is used in making pigments.