The material that forms a covering over the crown of a tooth (i.e. the part that projects above the gum). Enamel is smooth, white, and extremely hard, being rich in minerals containing calcium, especially apatite. It is produced by certain cells (ameloblasts) of the oral epithelium and protects the underlying dentine of the tooth. Enamel may also be found in the placoid scales of certain fish, which demonstrates the common developmental origin of scales and teeth.