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.
In more detail
A singly refractive stone stays dark as it turns. A doubly refractive stone blinks: four light and four dark positions per turn. An aggregate stays light at every turn. Down its optic axis, a doubly refractive stone also stays dark, so test more than one direction.
How we measure it
Our Polariscope computes the view by Jones calculus at 81 wavelengths, from each stone's refractive indices and measured absorption. It also has a conoscope, a red plate and an illustrative diamond strain. It is a teaching model, not a test of a real stone.
Related terms
- 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.
- 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.
- Conoscope: A conoscope is a strain-free glass sphere set on a stone in a polariscope. It gathers light from many directions at once, so the eyepiece shows an interference figure.
- First-order red plate: A first-order red plate is a retarder in a polariscope that shows which of a stone's two rays is slower. It turns parts of the interference figure blue or yellow, and that gives the optic sign.
- 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.