Our friends Lenore and Windell created an attachment to engrave glass, stone, and ceramic with an Eggbot.

The engraving tip itself is a small single-point industrial diamond, mounted on a thin steel shaft, in a fixture that holds it where the pen would normally be. That fixture consists of a 3/8-16 nylon thumb screw, drilled out to fit the steel shaft, and a matching 3/8-16 hex nut that locks it in place.