Marcasite is an Iron sulphide mineral similar to Pyrite, but with a different crystal structure. It is less stable and more brittle than Pyrite.
The word Marcasite can often refer to faceted Pyrite pieces used for jewellery – unfortunately, the words use in jewellery making pre-dates its strict definition as a mineral.
Faceted Pyrite pieces being referred to as Marcasite have been used in jewellery for centuries, especially popular in the 18th and 19th century. The pieces are still supplied, as they are often required for vintage jewellery restoration.