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.
In more detail
A singly refractive stone stays dark between crossed polarizers at every turn. A doubly refractive stone also stays dark when you look down its optic axis. So dark alone does not prove single refraction. Strain can make a singly refractive stone show patchy light.
How we measure it
The Polariscope shows diamond and cubic zirconia staying dark at every turn and tilt. Our build checks that no light passes them.
Related terms
- Doubly refractive: A doubly refractive stone splits light into two rays that vibrate at right angles and travel at different speeds. Each ray has its own refractive index.
- Polariscope: A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.
- Strain: Strain is internal stress that makes a singly refractive stone weakly doubly refractive in patches. Between crossed polarizers it shows as patchy light that never goes dark all at once.
- Crystal system: A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.