The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc = arcsin(1/n).
In more detail
A higher index gives a smaller critical angle. Diamond's is 24.4°, moissanite's 22.2°, cubic zirconia's 27.7°, ruby's and sapphire's 34.6°, emerald's 39.4° and amethyst's 40.4°. A refractometer uses a different critical angle: the one in its glass, against the stone.
How we measure it
Our gemstone pages give each stone's critical angle from the lower end of its refractive index. The head-on test on each cut page checks which pavilion facets lie at or above it.
Related terms
- 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.
- Total internal reflection: Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the light reflects back into the stone.
- Guide minimum pavilion angle: The guide minimum pavilion angle is the shallowest pavilion at which a round still returns head-on light through the table. It is the larger of the critical angle and (180° − critical angle) ÷ 4.
- Head-on light: Head-on light is light that enters a stone straight down along its axis. Our head-on test gives the share of the pavilion whose facets are steep enough to reflect that light.
- Windowing: A window is a face-up area through which you can see straight through a stone. It forms where pavilion facets are too shallow to reflect light, so light passes out of the back.