The hard enamel layer of teeth is composed of Calcium Hydroxy Apatite.

Explanation:
Enamel is the outermost and hardest layer of the tooth. It covers the crown of the tooth and protects the underlying dentin and pulp. The main component of enamel is a mineral called hydroxyapatite, which is a form of calcium phosphate.

Enamel is formed during tooth development, specifically during a process called amelogenesis. Ameloblasts, which are specialized cells, secrete the enamel matrix, which primarily consists of hydroxyapatite crystals. These crystals are made up of calcium, phosphate, and hydroxide ions.

The hydroxyapatite crystals in enamel are arranged in a highly organized pattern, which gives enamel its characteristic hardness and strength. This organized structure allows enamel to resist the forces of biting, chewing, and grinding.

Enamel is also highly mineralized, with a mineral content of around 96%. The remaining 4% is composed of water and organic material. The high mineral content of enamel contributes to its hardness and durability.

Enamel is non-living and does not contain any cells or blood vessels. It is also translucent, which allows the underlying dentin to show through. The color of enamel can vary from person to person, ranging from shades of white to yellow or gray.

However, despite its hardness, enamel is still susceptible to damage. It can be eroded by acids from foods and beverages, leading to tooth decay. It can also be worn down by excessive grinding or clenching of the teeth, a condition known as bruxism.

In conclusion, the hard enamel layer of teeth is composed of calcium hydroxyapatite, a mineral that provides strength and hardness to the tooth. Its highly mineralized structure and organized arrangement of hydroxyapatite crystals make enamel one of the hardest substances in the human body.