Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)).

The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x):[2] Hydrogrossular is a garnet variety in which a Si4+ is missing from a tetrahedral site.

Charge balance is maintained by bonding a H+ to each of the four oxygens surrounding the vacant site.

Hydrogrossular is found in massive crystal habit, sometimes grown in with idocrase.

Sources for green and pink hydrogrossular are South Africa, Canada, and the United States.