Angles for Cutting Garnets

Cutting Garnets Shallow

The color saturation of a gem depends directly on how long a distance light travels through it.  You can easily darken the color of a gem by making it deeper. Or lighten it by keeping it shallow.

Just how shallow is the question here. Some garnets come in large sizes. You might need to play with the angles quite a bit. Most garnets have a refractive index (RI) between 1.74 (pyrope and grossular) and 1.8 (almandine and malaya). This gives them a critical angle of 35 to 33°.

Cutting on the Critical Angle

The angle for maximum brilliance of a colorless stone in this RI range would be 42°. However, on a large garnet, that angle would make the stone so deep that the color absorption would eat much of the brilliance. The larger the gem and the greater the saturation, the closer you want to get to the critical angle.

On large gems with deep saturation, cutting right on the critical angle is not only acceptable but actually desirable. As the gem is moved, you get flashes of brilliance mixed with windowing. This windowing allows the color of the gem to…