Metallic silver permanent Sharpie marker pen.
We've found that there's just something about the silver colour that seems to give it a bit more staying power than other makes or colours. It's ideal for marking up your glass when cutting with water fed tools such as a Taurus ring saw.
Writes on glass but washes off with a little scrubbing. Permanent on a variety of other materials. No studio should be without this useful piece of equipment.