A tetragonal crystal.
Crystals have a mystery that has attracted people to them since ancient times. There are many different types of crystals that are considered valuable around the world. They are known for their beauty and rarity, and have interesting physical and chemical properties. Nowadays, their mystery is enhanced by the fact that crystals are a central theme in movies and games. Crystals are believed by some people to have healing powers and health benefits.
History
The word for crystals comes from the Greek word "krustallos" meaning ice. It was an ancient belief that a crystal was a type of ice, so cold it never melted. In prehistoric times they were used as jewelry, amulets and other such tokens. Egyptians used crystals for their healing powers. Native Americans and Indians also believed that crystals held spiritual powers. The Chinese and Japanese believed quartz was the soul of dragons and jade the essence of love.
Shapes
One of the ways scientists classify crystals is by physical appearance. Triclinic crystals are nonsymmetrical. Cubic crystals are cube-shaped, but they can also appear dodecahedral or octahedral. Tetragonal crystals come in prisms or double pyramids usually, but they are longer on one axis. Monoclinic crystals also come in double pyramids and prisms, except they are more like skewed tetragonal crystals. Orthorhombic crystals are a type of tetragonal crystal that forms rhombus prisms or two pyramids stuck together. Hexagonal crystals are six-sided, even viewed in cross section. Trigonal crystals have a threefold axis of rotation, unlike hexagonal crystals which have a sixfold axis.
Properties
Crystals are also classified in four different ways based on their physical and chemical properties. Covalent a covalent chemical bond through all the atoms in the crystals, resulting in high melting points. In metallic crystals, the metal atoms are arranged in a repeating pattern. They have high and densities. Ionic crystals, have their individual atoms held together by electrostatic forces. They also have a high melting point. Molecular crystals have clear molecular shapes in them but are held together with weaker bonds between their atoms. They tend to be soft and have low melting points.
Usage
The most common usage of quartz crystals is in watches as crystal oscillators. Watches use their mechanical resonance to a piezoelectric material to produce an electrical signal with a precise frequency used to track time. They are also used to stabilize frequencies for receivers or transmitters. The quartz crystals are also used in computers, cell phones, radios and other such devices.
Formation
The process of forming crystals--crystallization--usually occurs in solutions, molten substances and gas. The crystal formation is dependent upon temperature and other operating conditions. In crystallization, the molten substance or the solute has to form clusters. The clusters then get formed into geometric shapes periodically, significantly defining the shape of the crystal. The clusters need to be stable so they can build upon each other to support crystal growth.
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