Pavilion Depth Percentage
Pavilion depth percentage is the distance from the bottom of the girdle plane to the culet. It’s expressed as a percentage of average girdle diameter, and it can be judged visually, or measured and calculated. Pavilion depth percentage is important to brightness, as well as fire.
When you look at a face-up view of a diamond with a pavilion depth of around 40 percent, part of the girdle often reflects through the table, especially if you tilt the diamond slightly.
In many round brilliants with pavilions shallower than 38 percent, the girdle reflection forms an unattractive gray ring under the table. The ring might also appear in stones that have both a pavilion depth around 40 percent and a large table. The effect is called a fisheye. It gives the stone a dull, flat look.
Most stones with pavilions deeper than 49 percent look dark in the center. A pavilion depth of 50 percent or more can produce a dark area under the entire table. This is sometimes called a nailhead.
To put this into perspective, it helps to remember that the difference between a pavilion that’s too shallow and one that’s too deep is about 10 percent. On a round brilliant, this corresponds to a difference of only a few degrees in the pavilion angle, which is the angle between the pavilion main facets and the girdle plane.
Some people use the pavilion angle instead of the pavilion depth percentage when they evaluate a diamond’s proportions. Pavilion depth percentage and pavilion angle are two different ways of looking at the same thing: The steeper the angle, the greater the depth percentage.
Round diamonds have almost the same pavilion angle all around, so they also have the most even light display. On fancy shapes, though, pavilion angles are more variable from one part of the stone to another. This causes variations in light display. One of the most common effects, especially in marquises, ovals, and pears, is the dark bow-tie, which looks just like its name. A cutter can balance light display in a fancy shape by cutting an elongated culet or changing the angle of its pavilion facets.
To increase the weight of finished emerald-cut diamonds, some cutters increase the pavilion angles of the middle tier of facets, and cut a larger pavilion. This changes the overall profile of the diamond. The result is called pavilion bulge. Its disadvantage is that it often reduces the diamond’s brilliance. A large bulge can make a diamond difficult to set, and it adds to the weight—and the cost—without adding to the diamond’s beauty.