Inkless pens are seen by many people as a new category, but it's really more of a comeback. In Roman times, the stylus used for marking clay tablets was often made of lead, and was also used to mark wood or papyrus, as the lead left a dark mark behind. Modern inkless pens are essentially the same idea, though the small lead-alloy tip is mounted on a body of other metals, making them rather stronger, and safer to use.
In use, they're like a very hard grade of pencil, writing a grey mark on paper, but without the tip wearing down perceptibly. Because the metal-alloy tip wears away so slowly, they should last for a lifetime of occasional use.