Both of these lustrous minerals are greatly enhanced by the play of light across their striated prism faces. Sphalerite is unusual in that it is isometric, but so complicated in crystal habit, that only some of its faces are striated in different ways while others are perfectly flat in the same single crystal. The striations are the result of juxtaposition of several isometric forms. On its cubic faces, parallel striations are caused by the juxtaposition of two negative tetrahedron faces. On the negative and positive tetrahedral faces there are triangular striations caused by the juxtaposition of positive faces on the negative faces and visa-versa.
Octahedral faces are generally free of striations in sphalerite.
It makes for a rather complicated crystal. The many different feldspars have lamellar twinning that causes their parallel striations. But their striations only show up on their cleavage faces and not on their crystal faces. Striations on the cleavage faces of feldspars is very diagnostic for this generally difficult to distinguish group of minerals. 1998,1999,2000 by Amethyst Galleries, Inc.