Contrast is the part of a face-up stone whose light comes from 75° or higher above the girdle plane. This zone lies within 15° of the line of sight, where the viewer's head is.
In more detail
The light map shows contrast in blue. These areas reflect the viewer's head, so in real light they often look dark. In a well-cut round they form a crisp, even pattern that gives the stone sharpness. Contrast is not leakage: leakage areas look out of the back of the stone.
How we measure it
The simulator's light map counts light from 75° elevation and above as contrast. The Brightness & Tilt Analysis and the Cut Grading Tool use the same zone.
Related terms
- Light Return: Light Return sorts each point of a face-up stone by the direction its light came from. Its four classes, direct, indirect, contrast and leakage, together cover the whole face-up stone.
- 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.
- Head shadow: The head shadow is the part of the overhead light that the viewer's own head blocks. In our tools, it is a cone around the line of sight in which no light counts.
- Leakage: Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.
- Bow-tie: A bow-tie is a dark band across the belly of an oval, pear, marquise or other elongated stone. It is not an inclusion or a color: the facets there send little light back to the viewer.