Tourmaline is a dichroic material, meaning it displays two distinctly different colors. One color is viewed through the "A/B" axis (flat side of the crystal) and the other color is seen through the "C" axis (lengthwise down the crystal).
When tourmaline is cut in a standard round brilliant, the color that reflects out of the crown will be a mix of the "A/B" and "C" colors. Cutting tourmaline can be problematic because the two axes can have conflicting colors. Tourmaline can also have an axis with an undesirable color, or the "C" axis can be black or dark.
If the “C” axis is dark or black, known as a closed "C" axis, or has a bad color, it is best to cut the stone in a rectangle or square shape. Orient the stone so the pavilion facets that are in line with the bad “C” axis can be cut at a high angle such as 63-72 degrees. The steep angles reduce the reflection of the undesirable "C" axis from transmitting back through the crown.
Although there is only one option for cutting the pavilion, you can use different design varieties on the crown. I like to cut tourmaline with a closed "C" axis in an opposed bar, cushion shape or standard cut corner emerald pattern.
The variety of shapes that can be cut is limited, but the color, and therefore the value of your stone, will be greatly improved.