The shadow edge is the line between the dark and the bright part of a refractometer scale. It falls at the refractive index of the stone.
In more detail
Above the edge the scale is dark, and below it bright. A doubly refractive stone shows two edges, with a half-bright band between them. A singly refractive stone shows one edge, which stays in place as the stone turns.
How we measure it
Our Refractometer places each edge exactly at the published index. Amethyst, for example, shows edges at 1.544 and 1.553.
Related terms
- Refractometer: A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.
- Over the limit (OTL): Over the limit (OTL) is the refractometer reading of a stone whose refractive index is higher than the contact liquid's. The scale stays dark down to the liquid line, so there is no number to read.
- 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.
- Refractive index: Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.