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.
In more detail
It is 38.9° for diamond, 39.5° for moissanite and 38.1° for cubic zirconia. It is 36.4° for ruby and sapphire, 39.4° for emerald and 40.4° for amethyst. Faceters usually cut a degree or two steeper, for light that arrives off the axis.
How we measure it
Each gemstone page gives the figure for a round design, from the lower end of the stone's refractive index. Fancy shapes vary, so every cut page has its own head-on test.
Related terms
- Critical angle: 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).
- Pavilion angle: The pavilion angle is the slope of the pavilion mains, measured against the plane of the girdle.
- 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.