Carbonates & Sulfates
Calcite
What it is
Calcium carbonate, the mineral that builds limestone, marble, and seashells. Calcite has more crystal forms than any other mineral — over 800 documented shapes. Optical calcite (Iceland spar) produces dramatic double refraction, showing two images through a single crystal.
How it forms
Forms in marine environments from shells and coral, in hot springs, in cave formations (stalactites and stalagmites), and in hydrothermal veins. Honey calcite gets its color from iron, and blue calcite from trace copper.
CaCO₃ (Trigonal)
