A crystal is a uniform body with a geometric lattice or orderly repeating pattern.
The name crystal comes from the Greek word for "ice", as it was believed that rock crystal was water eternally frozen. "Rock crystal" originally described a clear quartz crystal. Optically clear does not mean it has no inclusions. It means that the crystal is not milky or cloudy at all.
Almost all metals can be found in a crystal formation. Most gems that are classified as precious started out as a crystal -diamonds, sapphires, rubies, etc. Though most people think of crystals as transparent, they can also be translucent or opaque. Nearly all minerals can grow in certain crystal forms. Some common "massive" minerals can form in crystals, but sometimes those crystals are very rare and valuable! Crystals will continue to grow, only by outside accumulation rather than internal growth. Usually this growth is extremely slow and takes thousands of years. Crystals can be microscopic to humongous. Gypsum crystals have been found in Nica, Mexico that are 40-60 feet long and 15 feet across! The larger a crystal is, the more likely it will have flaws or inclusions.
Most crystals are not regularly shaped, but have an irregular form, because some crystal faces have developed better and are more pronounced than others. However the angle between the faces always remains constant. Thus all crystals can be classified within the Crystal System in which crystallography has determined seven types of crystal growths based on imaginary axes and the different angles at which these axes intersect. The seven crystal systems are: Isometric, Tetragonal, Hexagonal, Trigonal, Orthohombic, Monoclinic, and Triclinic. Minerals do have a "habit" and will usually always form within the same crystal system.
When two or more crystals are united they are known as twin crystals. A double terminated crystal has a point on both sides. A enhydro crystal has water trapped inside. If there is a crystal growth within the crystal that is called a phantom. A crystal that has replaced a previous crystal and is therefore in a shape not known for the second mineral is called a pseudomorph.