From Dwarf Fortress Wiki
Pyrites are unremarkable low-value gems that can be found just about anywhere. In real life, it's most commonly known as fool's gold.
The base is usually white, but opals change depending on how they're viewed. [INORGANIC:PYRITE] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [MATERIAL_VALUE:2][DISPLAY_COLOR:6:7:1][TILE:15][IS_GEM:pyrite:STP:OVERWRITE_SOLID] [ENVIRONMENT:ALL_STONE:CLUSTER_SMALL:100] [SOLID_DENSITY:4890] [STATE_COLOR:ALL_SOLID:SILVER]