I have found the by holding the small stone, that have been glued to
nail heads for cutting and polishing, with a small piece of soft
leather gives me more leverage and also protects my fingers from
possible cuts as the stone and nail separates. Maybe because my
fingers are protected I am able to use more force to break the bond.
Try it and see if it works for you. Usually on the larger stones I
don't need or use any protection but with the smaller stones it sure
seems to help.
Larry Still teaching 7th and 8th graders and polishing stones when
I have time.