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.
In more detail
Many diamonds carry strain from growth. A truly doubly refractive stone goes dark everywhere at once, four times per turn. So patchy light points to strain. In GIA's study, moissanite showed no strain.
How we measure it
Our Polariscope offers an illustrative strain for diamond only, with path differences up to 140 nm. The direction of the slower ray wanders through every angle. The pattern is invented to show the effect, not measured from any stone.
Related terms
- 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.
- Blink (polariscope): A stone blinks in a polariscope when it turns light and dark four times per turn between crossed polarizers. Blinking shows that the stone is doubly refractive.
- 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.
- 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.