Calcite

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.