True, but among the minerals with cubic crystal structure it is not unusual for them to be found as crystals that are perfect regular or semiregular polyhedra, with a shape characteristic for the mineral, for instance octahedron (e.g. spinel, diamond), rhombic dodecahedron (e.g. garnet) or cube (e.g. pyrite).

I suppose that the crystals from the picture are of pyrite, which frequently looks like this.

In the antiquity, when what are now called diamonds (the Romans and the Greeks called them "Indian adamants", because they were first encountered by Europeans during the expedition in India of Alexander the Great; "adamant" meant something else in Europe) were very difficult to cut and polish, they were normally used as gems in their natural shape of regular octahedra.

Cutting diamonds from their natural octahedral shape into polyhedra with more facets, e.g. brilliant, was invented much later.