Slate is a dark, metamorphic rock that forms when shale is exposed to the proper conditions of heat and other minerals. Slate can be grey, black, green and a variety of other shades. Carving shale requires the proper tools, but fortunately most of these are standard stone carving tools.
One of the basic slate carving tools is a chisel. Chisels are shafts of metal with one end that's flared and sharpened to a carving edge. The other end of the chisel is flat and solid because it's meant to be struck with a hammer. Chisels come in a variety of sizes, from large, heavy duty chisels that are meant for rough work, to smaller chisels which are meant to be used for detail work on stone sculptures. The hammers meant for striking these chisels also come in a variety of appropriate sizes and weights.
Drills are used in a variety of different ways for carving stone, including slate. Drills can make round score marks on the stone that can be used for guiding points while creating more detailed. Drills can also be used to make holes for chisels to split open slate. Additionally, drills come in regular variety and hammer variety. Hammer drills are partially or wholly pneumatic, and in addition to spinning in a circle, there's a hammer effect that pushes the drill bit down into the stone.
Another tool used to help carve slate (in addition to other types of stone) is a grinder. A grinder is a machine with a head that spins in a circular motion. The head is covered in a rough, abrasive material that sands away the slate one layer at a time as it's pressed against the stone. Grinders are used to take sharp edges off of the slate, or to smooth out grooves on the stone when the carving is getting close to its final stages.