Crystals form through a process called crystallization. This can occur in a variety of ways, such as the hardening of a cooled liquid, or water evaporating from a substance. The molecules found in crystals have a repeated pattern, and crystals can form into different types of geometric shapes.
Some common crystal shapes include triangles, rectangles and squares. Crystals also can be classified into six different groups, which are cubic, triclinic, monoclinic, hexagonal, orthorhombic and tetragonal. Cubic crystals have 90 degree angles; diamonds and garnets are considered types of cubic crystals. Crystals with a hexagonal shape include aquamarine and sapphire, while rutile peridot and topaz are classified as orthorhombic crystals.
Rock crystals tend to form when rock is heated, turns into magma, cools down and then hardens. There are many examples of crystals in nature, such as salt, sulfur, gypsum and sugar. Snowflakes are ice crystals that also are seen in nature.
Crystallography is the study of crystals. Scientists believe that zircon crystals found in western Australia are the oldest, dating back nearly 4.4 billion years. The largest diamond, called the Cullinan diamond, was found in South Africa in 1905. Its weight was estimated at 3,106 carats.
Many different types of crystals are used for their purported spiritual and healing properties. For example, certain crystal are associated with different chakras in the body and are purportedly used for balancing these energy centers.