Step 1: Recall what tooth enamel needs to do.
Enamel is the hard, protective outer coat of a tooth, so its job needs it to be extremely hard and mineral rich rather than soft and organic.
Step 2: Identify the mineral responsible for that hardness.
About 96 percent of enamel is made of a crystalline mineral called hydroxyapatite, a form of calcium phosphate. \[ Ca_5(PO_4)_3OH \] This tightly packed calcium and phosphate structure is what gives enamel its rock like hardness.
Step 3: See why this matters for tooth decay.
Because hydroxyapatite is calcium phosphate, it reacts with acid when the mouth's pH drops below about 5.5, which is exactly why sugary food, fermented by mouth bacteria into acid, damages enamel over time. This decay chemistry only makes sense if enamel is a calcium phosphate mineral to begin with.
Step 4: Conclude.
Tooth enamel is made of calcium phosphate. \[ \boxed{\text{Calcium phosphate}} \]