An aluminum marking pencil is a great asset when used to mark rock slabs with a template pattern for cabochons.  The mark made with the aluminum pencil will not come off with water while you are shaping your cab.  When and if you need to change your pattern, just use a little powdered cleanser.  The pencil is 5 3/4" long. Having two on hand makes it easy to find at least one handy when you need it.