The mineral is calcite.
Lustre — glassy; ∴ non-metallic
Hardness — between a fingernail (2.5) and a penny (3)
Streak — white
Specific gravity — 54/20 = 2.7
Colour —white
Acid reaction — Fizzes; ∴ a carbonate
The only carbonate mineral that satisfies all these criteria is calcite, CaCO₃.
