From Dwarf Fortress Wiki
Black pyropes are semi-valuable gems that can be found in metamorphic layers as well as within kimberlite. The other variety of pyrope found in Dwarf Fortress is red pyrope, which is of equal value and can be found in the same environments.
[INORGANIC:BLACK PYROPE] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [MATERIAL_VALUE:20][DISPLAY_COLOR:0:7:1][TILE:15][IS_GEM:black pyrope:STP:OVERWRITE_SOLID] [ENVIRONMENT:METAMORPHIC:CLUSTER_SMALL:100] [ENVIRONMENT_SPEC:KIMBERLITE:CLUSTER_SMALL:100] [SOLID_DENSITY:3900] Common to garnets. Range is 3500 - 4300 [STATE_COLOR:ALL_SOLID:BLACK] type of andradite, a garnet