|North America||Bingham, Utah||6 ½" x 4 ¼" x 1 ½"|
A crystal form is a set of equivalent crystal faces. Equivalent, in this case, does not mean absolutely identical, but equivalent in terms of the crystal's symmetry. The vagaries of Mother Nature's crystal-growing methods may result, for instance, in what should be perfect cubes being, instead, flattened cubes. The two most common forms for pyrite are cubes and pyritohedrons. The label for this specimen says these are pyritohedrons, but alas, they are cubes, as you can see.
As the label indicates, these cubes are striated - they show th in, parallel grooves on each face, and on adjacent faces, the striations are in different directions. What would make these grooves? It is alternating growth of the cube and pyritohedron forms. The crystal, for whatever reason, begins growing the cube form, then switches to the pyritohedron form, then alternately switches back and forth. On the drawing of the pyritohedron, notice the vertical edge between the two slightly irregular pentagons in front. That is the edge to which the striations are parallel on one cube face. The corresponding edges on the top and right side of the drawing "point" in different directions, accounting for the directions of the striations on the other cube faces.
Striations on minerals are not rare, but not all are caused by alternation of two crystal forms. Some are due to a phenomenon called twinning.