Macle

 

A term used in the diamond trade for a flat, triangular, rough diamond, which is a twinned crystal of the spinel-twin type (in which the twin plane is parallel to an octahedral face, with one side appearing to have been rotated 180 degrees with respect to the other). It is more difficult to fashion than most other crystals because of the differing grain directions caused by the twinning. Due to their odd shape, macles are often used for fancy cuts.