Nailhead is a trade term for a poorly cut diamond that looks dark on the table.
In more detail
The term names a look, not a measured value. In GIA's cut grading examples, a very steep pavilion and a large total depth give a very dark table. GIA's cut research counts large dark areas in the face-up pattern as a negative trait.
Related terms
- Fish-eye: A fish-eye is a dull ring that shows under the table of a round diamond. It is a reflection of the girdle, seen when the pavilion is too shallow.
- Leakage: Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.
- 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.
- Pavilion depth: Pavilion depth is the depth of the pavilion, from the girdle to the culet. It is a percentage of the average diameter.
- Cut grade: The cut grade rates how well a diamond's proportions and finish work, from Excellent to Poor. GIA gives it only to standard round brilliants in the D-to-Z color range.
- Contrast (Light Return): 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.