Marcasite

Marcasite is an iron sulfide mineral (FeS₂) sharing the same composition as pyrite but forming in a different crystal structure, typically as pale brass-yellow to white flat tabular or cockscomb crystal aggregates with a metallic luster. It forms at low temperatures in sedimentary environments and is found in clay deposits, chalk beds, and coal measures across Europe and North America, though it is notorious for instability and eventual decay. Faceted or set marcasite in Victorian jewelry typically refers to small polished pyrite table-cut stones and became fashionable in silver settings during the 19th century.