Octahedron

These have forms such as the cube, octahedron, etc., which belong to the isometric system, and it is in this system that the Diamond crystallizes.

Twinned crystals, in which the two individuals may be connected by a single plane or may be interpenetrating, a star shape often resulting in the latter case, are common; sometimes, if of the octahedron type, they are beautifully symmetrical.