The main constituent of enamel on the surface of teeth is:

Enamel provides the hard surface to the teeth. It has the carbonate substituted hydroxyapatite crystallites. It is formed from the minerals. It has the calcium phosphate and calcium hydroxide.

Hence, the main constituent of enamel on the surface of teeth is $\left[3{\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2·\mathrm{Ca}(\mathrm{OH}{)}_2\right]$.