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What is a Crystal?
by International Gem Society
“Crystal 1” by Brenda Clarke. Licensed under CC By 2.0.
"What comes to mind when you think of crystals? Many people might visualize beautiful, mineral objects with smooth faces in regular geometric patterns. Others might imagine elegant glassware.
For gemologists, the scientific definition of a crystal 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 systems. If a mineral has its atoms arranged in one of them, then that mineral is a crystal.
Crystal Systems
There are seven crystal systems: isometric, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal. Each is distinguished by the geometric parameters of its unit cell, the arrangement of atoms repeated throughout the solid to form the crystal object we can see and feel.
For example, an isometric or cubic crystal has a cube as its unit cell. All its sides are equal in length and all its angles are right angles. Well-known gems in this system include diamonds, garnets, and spinels.
On the other hand, a triclinic crystal has all sides of different lengths and none of its angles are right angles. These geometric variations mean triclinic crystals can take on many intricate shapes. Well-known gems in the triclinic system include labradorite and turquoise.
Non-Crystalline Solids
Some objects may appear to be crystals to the naked eye, but outward appearances can be misleading. ... "
https://www.gemsociety.org/article/crystal/
