Tricalcium phosphate.

Fossil bones and teeth are composed of calcium phosphate. At a pH of 7, bones and teeth will not dissolve. Calcium phosphate is slightly soluble in acidic conditions and so it is recommended to add calcium phosphate as a buffer during acid preparation. By adding calcium phosphate to dilute acid, you 'use up' the ability  of the acid to dissolve bones and teeth. Carefully mix in until no more will dissolve. 

Dilute acid to the required concentration and stir in calcium phosphate until no more will dissolve. 

Vegans and Vegetarians: This calcium phosphate is not made from animal products or by-products.  It is not fit for human consumption, but you might feel more comfortable using it!

Safety Data Sheet