This is a photo of a pyrite cube, possibly from Washington State. The side-lighting in the photo highlights the topographic-like striations on the surface, while the black areas are reflecting the camera lens/possibly the room (the shutter speed is fast enough to eliminate any environmental light). The edge at the top of the photo is irregular as a chunk of the specimen was broken off at some point (or maybe it was part of a larger cluster).
Approximate Photo Location (Side 1)
Field of view: ~5/16” x 7/16“ (7.4mm x 11.1mm)
Images in focus stack: 30
From what I can tell, the striations on the face are caused by the crystals alternating between cubic (square) and pyritohedral (pentagon) forms as they grow (1, 3). The illustrations below show the different orientations of the striations. It is kind of interesting that their overall direction is perpendicular to the edge of the cube and they run in three directions, where the front face of the cube matches the back face, the top face matches the bottom face, and the right face matches the left face (2, 3).
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