Forms in low-temperature hydrothermal veins, often with zinc and lead sulfides. Like pyrite, it is used for the extraction of sulfuric acid. Because of its bright luster it used to be cut for trinkets. From Alabama.

M906 - Marcasite Nodules, Alabama 

Marcasite is a Iron Sulfide. Hardness 6 to 6-1/5. Marcasite appers in a number of crystal formations, the concretionary or nodular form is probably the most common.

The mineral marcasite is iron sulfide and is sometimes called white iron pyrite. Marcasite is often mistakenly confused with pyrite, but marcasite is lighter and is more brittle. Marcasite often crumbles and breaks up due to the unstable crystal structure. The main difference between marcasite and pyrite is this crystal structure. Though marcasite and pyrite have the same crystal chemical formula, marcasite crystallizes in a different crystal system, thereby making it a separate mineral.

On fresh surfaces, marcasite is a pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and give a black streak. Marcasite is a brittle material that cannot be scratched with a knife. Hardness 6 to 6-1/2.

A member of the Sulfides group of minerals.
In the making of jewelry it is said that marcasite is used to make marcasite crystals, when in actually true marcasite is never used in making of "marcasite crystals". Pyrite is used in the making of these crystals for jewelry, never true marcasite. Marcasite is too unstable of a mineral to be used for this purpose. So, the "marcasite jewelry" is a mis-nomenclature. It is actually pyrite.