A carborundum stone is a abrasive stone used to grind away and finish sharp edges of glass pieces by hand. This can be used if you don't have an electric glass grinder, but that is a very labor-intensive way to get the job done on any but the smallest pieces.

Even if you do have an electric glass grinder, it's good to have a carborundum stone on your workbench for the small jagged edges you will occasionally run across after grinding that don't deserve firing up the grinder again. 

Carborundum stones are sometimes difficult to find and may not be available at all stained glass supply stores. When you do find them, they run around $7.