A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.
In more detail
Diamond and cubic zirconia are cubic, so they are singly refractive. Beryl and moissanite are hexagonal; corundum and quartz are trigonal. These stones are uniaxial, so they can show pleochroism and blink in a polariscope. Some references count trigonal as part of the hexagonal system.
How we measure it
Our gemstone pages give each stone's crystal system and show a unit cell of its structure.
Related terms
- Singly refractive: A singly refractive stone has one refractive index in every direction, so light stays one ray inside it. Diamond and cubic zirconia are singly refractive.
- Uniaxial: A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.
- Biaxial: A biaxial stone is a doubly refractive stone with two optic axes. It can show three colors in a dichroscope, and its interference figure differs from a uniaxial one.
- Birefringence: Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.
- Pleochroism: Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.
- Cleavage: Cleavage is a mineral's tendency to split along flat planes in its crystal structure. A hard knock in a cleavage direction can split a stone.