Crystal Systems
The seven crystal systems and their characteristic symmetry elements.
Overview
All crystals belong to one of seven crystal systems, classified by their unit cell
geometry and symmetry elements. Understanding these systems is fundamental to
crystallography and gem identification.
Crystal systems are defined by the relationship between crystallographic axes
(a, b, c) and the angles between them (α, β, γ). Each system has characteristic
symmetry elements that determine possible crystal forms.
Symmetry Elements
Rotation Axes
2-fold, 3-fold, 4-fold, 6-fold rotational symmetry
Mirror Planes
Reflection symmetry planes
Center of Symmetry
Inversion through a point
The Seven Systems
The highest symmetry system with four threefold axes.
Characterized by one sixfold axis.
Characterized by one threefold axis.
Characterized by one fourfold axis.
Three twofold axes or two mirror planes.
One twofold axis or one mirror plane.
Lowest symmetry - no rotational symmetry.
Identifying Crystal Systems
When examining a crystal or gemstone, you can often determine its crystal system
by observing:
- Crystal habit - The external shape and face arrangement
- Optic character - Isotropic (cubic) vs. uniaxial vs. biaxial
- Interference figure - Under polarized light microscopy
- Cleavage directions - Related to internal symmetry
Quick Reference
| System | Optic Character | RI Values |
|---|---|---|
| Cubic | Isotropic | Single RI (n) |
| Hexagonal, Trigonal, Tetragonal | Uniaxial | Two RI values (ω, ε) |
| Orthorhombic, Monoclinic, Triclinic | Biaxial | Three RI values (α, β, γ) |
Common Gemstones by System
Cubic
- Diamond
- Spinel
- Garnet (almandine, pyrope, grossular)
- Fluorite
Trigonal
- Quartz (amethyst, citrine)
- Ruby
- Sapphire
- Tourmaline
Hexagonal
- Emerald
- Aquamarine
- Morganite
- Apatite
Orthorhombic
- Topaz
- Peridot
- Chrysoberyl
- Tanzanite