Calcite (CaC03) occurs in many environments and, while its crystal system
is trigonal, there are many crystal manifestations. The one constant is
perfect rhombohedral cleavage. Calcite is the standard for hardness 3 on
the Mohs Scale.

Calcite is an important raw material and is used in a variety of industries
from building materials to chemical manufacturing

This is a calcite cleavage fragment. it is about 3 x 1 x 3/4 inches. Because calcite's high index of refraction and cystral system, even small pieces display double refraction.