Calcite (CaCO₃) is one of the most common minerals on Earth, forming in sedimentary and hydrothermal environments. Calcite crystals can range in color from milky white to emerald green and deep blue. Known for its birefringence, it plays a key role in the formation of caves and karst formations. It is also used in crystal therapy to promote mental clarity. Main sources: Mexico, Brazil, China, the United States, Italy, and Morocco.