Striated Iron Pyrites cube

"Striations are a product of a mineral's crystal habit. They can be very diagnostic and can even be an enhancement to the attractiveness of a specimen. They are incorrectly referred to as scratches, especially on transparent crystals.

The most common cause of striations are the convergence or juxtaposition of two crystal faces. One of the faces gets truncated or "overtaken" by the other, but manages to leave its mark in the form of an almost imperceptible edge or stria. This edge is repeated again and again as the mineral grows and can fill an entire face with these tiny edges or striations.

Generally striations are parallel, but some are triangular or even crossed. Striations are generally common, but the best examples of striations are on the minerals quartz, pyrite, apophyllite, tourmaline, sphalerite and the feldspars, but there are many others.

Pyrite has striations on its cubic faces that are caused by faces that are not usually on the crystal at the time. Pyrite's striations are caused by a juxtaposition of two pyritohedron faces on the cube face. Generally the pyritohedron faces are not even formed on the typical cube habit, but their striations are there as straight lines. It is interesting to notice that the striations on one side of the cube are perpendicular to the striations on the other side (shown in the photo below)."

www.galleries.com/minerals/property/striatio.htm

en.wikipedia.org/wiki/Pyrite