Pure calcite is clear in color. This orange calcite sample is colored because it's got iron impurities. Calcite is very common. It's the main constituent of reefs, cave formations, limestone, and marble. It's uses vary from an agricultural soil treatment to the manufacture of steel, cement, and glass. It's soft enough (hardness level of only 3) to be used as an abrasive for most other rocks and metal. It's the same stuff in Michelangelo's Pieta and the antacid you take for heartburn. Plus - get this - in all experiments to make a cloak of invisibility (there are groups trying to do this: University of California - Berkeley among others) calcite is an important ingredient.