Pyrite crystals from Spain. Image credit: Christies.
The mineral pyrite is the most common form of iron sulfide in nature. It forms cubic crystals. Small cubes sometimes aggregate into spheres to form a pyrite concretions. Pyrite concretions catch people’s attention because they are round and dense.
Spherical pyrite concretions. In some of the photos, if you look hard, you can see the cube corners sticking out. On the reddish ones the iron sulfide has oxidized to the iron oxide hematite. If you saw or break them in two, you can see the metal-like pyrite (lower right).