Light and shade on a grey background. You can guess at the origin of the patterns but there's something nicely deceptive about the way the lines meet, the fact that the various parallelograms touch at their corners, but not quite, the light grey parallelograms almost line up with the dark ones, but not quite. It's odd to think that (crystal structures apart) this type of image is entirely artificial, it just doesn't exist in the natural world.

This is a different view of the same thing - different light, different angle.