A usually dark brown- or gray-colored nozzle used for sandcarving. It is typically made from an alloy of elemental boron and metal that makes it similar to steel, but much harder. The makers of these nozzles list their life span of continuous blasting at about 200-plus hours. Cost of such a nozzle is between $120 and $180 each.
Seen in the image are a few different nozzles. Left: Ceramic nozzles; middle: tungsten carbide nozzle; right: boron carbide nozzle.
Information and image courtesy Ruth Dobbins