Marcasite
The mineral marcasite, sometimes called white iron pyrite, is iron sulfide. Marcasite is often mistakenly confused with pyrite, but marcasite is lighter and more brittle.
Marcasite has been known to the world since the beginning of civilization. For instance, the Greeks used it as a talisman and the Chinese mined it quite frequently. Mayans and Aztecs also used the stone, which today is still used to create certain types of ornaments.
The name of Marcasite has been very popular, especially when associated with silver, however for the most part, when most people are referring to Marcasite in silver and other gems, they are actually referring to Pyrite with is a polymorph of Marcasite.
Polymorphs are very interesting, a polymorph of a mineral is when two minerals are made of exactly the same chemical compounds; however they are arranged or structured differently. This structure might have a lot of similarities, but the two minerals are ultimately different. Another common polymorph are diamonds and graphite.