Gabbro is an igneous intrusive magma-safe stone which can form whole layers.
It is the only stone that contains kimberlite, which in turn is the only stone that contains diamonds, and garnierite, the only ore of nickel. There are several gems unique to gabbro, as well.
Gabbro is also left behind by a defeated gabbro man.
In real life
Gabbro is the rock that forms layers on oceanic plates. Some variants are known by the trade name "black granite," mainly for countertops and gravestones. It is otherwise unremarkable to the layman.