Calcite is a common carbonate mineral and when pure is a 3 on the moh's scale of hardness. Calcite is composed of calcium and is acid sensitive and a great stone for carving! 

Calcite comes in almost every color and is mined for it's beauty and both is commercial and household use. 

Calcite is a primary ingredient in limestone and has a waxy feel when in crystal form. Calcite is a great choice for jewelry which will not be subject to a lot of wearing, like ring stones. Much better for statues and pendants. 

Many stones with the name "onyx" are often a banded calcite. 

Calcite forms tons of different crystal structures and some really big and beautiful ones. Calcite can also form in one big massive crystal. 

Calcite when subject to the heat and pressure of metamorphosis becomes marble.