What is a Crystal?

What comes to mind when you think of crystal? Many people might visualize a beautiful, mineral object with smooth faces in regular geometric patterns.

“Crystal 1” by Brenda Clarke is licensed under CC By 2.0

Others might imagine elegant glassware.

“Crystal” by liz west is licensed under CC By 2.0

For gemologists, there is a scientific definition of a crystal that goes right to the atomic level. A crystal is a solid whose atoms are arranged in a “highly ordered” repeating pattern. These patterns are called crystal lattices. If an object has its atoms arranged in one of seven crystal lattice patterns, then that object is a crystal.

There are seven crystal lattice patterns: cubic, tetragonal, rhombohedral (or trigonal), hexagonal, orthorhombic, monoclinic, and triclinic. Each is distinguished by the geometric parameters of its unit cell, the arrangement of atoms that is repeated throughout the solid to form the crystal object we can see and feel.

Crystal Lattices
“Cubic,” “Tetragonal,” “Rhombohedral,” “Hexagonal,” “Orthorhombic,” and “Monoclinic.” Original PNGs by Daniel Mayer, traced in Inkscape by User:Stannered – donated work. Licensed under CC BY-SA 3.0. “Triclinic.” Original PNGs by DrBob, traced in Inkscape by User:Stannered – donated work. Licensed under CC BY-SA 3.0/Images cropped from originals and rearranged into a single pic to facilitate illustration.

For example, a cubic lattice has a cube as its unit cell. All its sides are equal in length and all its angles are right angles. A triclinic lattice, on the other hand, has all sides of different lengths and none of its angles are right angles. These geometric variations mean crystals can take on many intricate shapes.

Some objects may appear to be crystals to the naked eye, but outward appearances can be misleading. For gemologists, the atomic structure of the object is what determines whether it is a crystal. Not all objects that have regular geometric faces are crystals, and not all solid materials are crystals. Glass, for example, has a non-crystalline, amorphous atomic structure. Although glass can be poured and hardened into a geometric shape, its atomic structure doesn’t change. (And while some glassware is commonly and contextually referred to as crystal, it is in fact not crystal, scientifically speaking).

Water that hardens into a single large snowflake is in fact a crystal. (Water crystallizes as it cools, freezes, and moves through the atmosphere). Water that hardens into a cube in your freezer’s ice tray, however, is not a crystal. Ice cubes, rocks, and common metals are examples of polycrystalline materials. They may contain crystalline objects (in the case of ice cubes, they may contain actual ice crystals) but the entire material or object cannot be described as having a uniform crystal lattice structure.

A snowflake is a crystal … “Snowflake-29” by Yellowcloud is licensed under CC By 2.0
… An ice cube is not. “Frozen Ice Cubes IMG_1021” by Steven Depolo is licensed under CC By 2.0

Most crystals have natural origins. They can form through inorganic means, such as geological processes within the earth, or organic means within living creatures. For example, some kidney stones are made of weddellite crystals.

Weddellite is a crystal that can be found at the bottom of the Weddell Sea near Antarctica. It can also be found passing very painfully through urinary tracts. “Surface of a Kidney Stone” by Kempf EK is licensed under CC By-SA 3.0

Crystals can also be created artificially. Cubic zirconia is a synthetic crystal with a cubic lattice structure that is created by super heating zirconium and zirconium dioxide. The result is a material that is difficult to distinguish from a diamond.

“Multicolor Cubic Zirconia” by Michelle Jo is licensed under CC By 3.0