This type of twinning produces a twin plane that is parallel to one of the octahedral faces. The plane acts as a mirror plane and produces a left and right side that are mirror images of each other. This may not sound all that spectacular for a very symmetrical mineral like spinel which is loaded with mirror planes. However this mirror plane is not parallel to any of the others and lowers the outward symmetry of the crystal. The crystal will now have a trigonal appearance with a flat triangular top and bottom.
Color is commonly green but also known to be blue, yellow, brown or gray.
Luster is vitreous. Crystals are generally just translucent to almost opaque, but some slightly transparent specimens are known. Octahedrons may be truncated by dodecahedral faces and commonly show striations parallel to the octahedron-dodecahedron boundary. Twinning by the Spinel Twin Law can produce trigonal appearing crystals with three reentrant angles on the sides of the crystal. None Fracture is conchoidal.