Crystals consist of numerous small building blocks made up of a particular arrangement of atoms. How these building blocks are combined determines the crystal's shape. The building blocks of a crystal combine to form a so-called atomic grid which gives a crystal its geometric shape or "symmetry. All crystals are symmetrical in one way or another.
Six fundamental crystal systems can be distinguished, each of which gives the shape based on the arrangement of the atoms or molecules.
Cubic or Isometric Crystal System
This is the most symmetrical and simplest crystal system. It has three equal axes and these are perpendicular to each other. Because of the equality of the axes, minerals in the cubic system are single refractive or isotropic. The main forms are cubes and octahedrons. Examples: Galena, Halite, Silver, Gold, Fluorite, Diamond, Pyrite, Lapis Lazuli, Sodalite, Garnet, Spinel, Magnetite and Copper.
Tetragonal
The tetragonal system is the least common system and is characterized by two axes that are equal in length and a third axis that is longer or shorter. Examples: Apophylite, Idocraas, Rutile, Vesuvianite, Chalcopyrite, Zircon, Cassiterite, Wulfenite and Scheelite.
Orthorombic
There are three axes in this crystal system, all of which meet at a 90-degree angle. However, all the axes are of different lengths. Examples: Barite, Olivine, Topaz, Hypersthene, Peridote, Iolite, Sulfur, Aragonite, Celestine and Cerussite.
Monoklien
Here all three axes are unequal in length and are not perpendicular to each other. A dual axis of symmetry. Examples: Malachite, Staurolite, Kunzite, Selenite gypsum, Mica, Orthoclase, Manganite, Hornblende, Borax, Azurite, and Diopsite.
Triklien
This is the least symmetrical system, with three unequal axes and no intersections at 90-degree angles. Examples: Microlene Feldspar, Amazonite, Aventurine, Albite, Turquoise, Serpentine, Kaolinite.
Hexagonal (and Trigonal)
Crystals in this system have a hexagonal system with an additional axis, giving the crystals six sides. Three of these sides are the same length and are 60 degrees apart. The vertical axis is at 90 degrees from the shorter axes. Examples: Apatite, Aquamarine, Morganite, Emerald, Beryl, Quartz.
Amorphous
In addition to these six crystal systems, there are also amorphous materials in which molecules do not fix themselves in a regular lattice. Amorphous materials are not technically minerals because they do not form in any of the crystal systems. Examples of amorphous materials used as gemstones include Amber, Obsidian, Git, Moldavite and Opal.