Sphalerite is a polymorph with two minerals, wurtzite and matraite. The three are called polymorphs. Which means they have the same chemistry, (Zn, Fe)S or zinc iron sulfide. But they have different structures and therefore different shapes. Sphalerite is the more common mineral of the three. There is also an iron rich variety of sphalerite that is called Marmatite. Marmatite is not a commonly seen mineral because it is brilliant black like black sphalerite. So it is very often unrecognized. Marmatite is chemically a ferroan sphalerite. It contains, up to twenty percent iron in addition to zinc sulfide. There isn't much more to say about this specimen. It's a nice old specimen. It is in very good condition, with sparkling little black crystals. It is from the Eagle Mine Eagle County, Colorado.