From Dwarf Fortress Wiki
It is the only stone that contains kimberlite, which is the only stone that contains diamonds. Gabbro is also the only stone to contain garnierite, the ore of Nickel. There are several gems unique to gabbro, as well.
[INORGANIC:GABBRO] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [STATE_NAME_ADJ:ALL_SOLID:gabbro][DISPLAY_COLOR:0:7:1][TILE:177] [IGNEOUS_INTRUSIVE] [IS_STONE] [MELTING_POINT:12160]