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.
In more detail
Windowing depends on the angle and the refractive index together. A pavilion that works in diamond can window in amethyst. Amethyst's critical angle (40.4°) is much larger than diamond's (24.4°).
How we measure it
On cut pages, the head-on test reads “Minor windowing” when 90% to under 99% of the pavilion reflects head-on light. The light map shows windows as leakage.
Related terms
- 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.
- 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).
- Leakage: Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.
- Light map: A light map is a face-up picture of a cut stone, colored by the direction each point's light came from. Red is direct light, green is indirect light, blue is contrast, and a dark color is leakage.
- 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.