This is the mineral Calcite. It’s made of 3 elements, Calcium, Carbon, and Oxygen, and has the chemical formula CaCO3. It’s often called Calcium Carbonate. It’s relatively soft and is the defining mineral for hardness rank number 3 on Mohs hardness scale. It often exhibits a vitreous, or glassy, luster, but is sometimes resinous, waxy, and even pearly. Calcite easily produces a white streak on a ceramic streak plate. It has a specific gravity of 2.7, so it feels relatively light in your hand. An especially distinctive characteristic of Calcite is its perfect rhombohedral cleavage. Well-formed specimens exhibit cleavage planes that intersect at about 75-degrees. Calcite belongs to the Carbonate chemical family. All Carbonate minerals contain a Carbonate ion. Carbonate ions are compounds made of one carbon atom surrounded by three oxygen atoms. Carbonate ions are unstable in the presence of Hydrochloric Acid. The acid reacts with the Carbonate ions to produce Carbon Dioxide in an effervescent, or bubbling, reaction. Geologists can thus test for Carbonate minerals using the acid test, which is conducted by placing several drops of Hydrochloric Acid on a specimen and watching for effervescence. Calcite is a common mineral that forms in hot springs, caverns, and coral reefs. It’s the main component in the shells of snails and clams. Since most of Earth’s water contains dissolved Calcium and Carbon Dioxide, when water evaporates, Calcite is often precipitated out of solution. This is especially visible caverns where stalactites and stalagmites are present. Calcite is the primary mineral in the sedimentary rock called Limestone. Limestone is used around the world for constructing roads, buildings, and monuments. Metamorphosed Limestone is Marble, which is another important rock used to create items as varied as bathroom sinks to Michelangelo's David sculpture. Cement production requires Limestone, and thus the mineral Calcite. Sometimes Calcite is added to soil to increase its Calcium content and to regulate the soil’s pH.