marcasite
noun
mar·ca·site ˈmär-kə-ˌsīt 
-ˌzīt;
ˌmär-kə-ˈzēt
1
a
: crystallized pyrite
b
: a pale yellow to white mineral of the same composition and appearance as pyrite but of different crystalline organization and lower specific gravity
2
: a piece of marcasite used in jewelry
Word History
Etymology
Middle English marchasite, from Medieval Latin marcasita
First Known Use
15th century, in the meaning defined at sense 1a