Calcite

(CaCO₃) the mineral (calcium carbonate) can be white to colorless, gray, and pale-yellow to honey-brown. It has a hardness of 3, a vitreous luster, and white streak. In calcite, three directions of well-developed cleavage (splitting along definite planes) form a rhombohedron, which looks like a distorted cube. A diagnostic feature of calcite is that it effervesces (bubbles) vigorously in dilute hydrochloric acid. Calcite crystals are commonly found lining open cavities in rock.

Calcite is one of the most common minerals in the upper part of the Earth's crust. Texas has abundant limestone formations, and limestone is made up primarily of calcite. Metamorphosed limestone, called marble, is also made up mostly of calcite. Caliche, the white soil present in many parts of Texas, is mostly finely crystalline calcite.

Calcite is commonly used in optical instruments and the manufacture of cement and glass.

The calcite sample in the Texas Rock Kit was collected in Travis County. The limestone in which the calcite formed is Cretaceous in age.