Pro-Quality Gold Test Stone- 6x3 inch large test stone Smooth Black Glass Surface for easy reading of the test acids on this stone. Non slip pad on back to keep stone from sliding or scratching on table surfaces Large 6x3 tesing surface make it easy to test several items at once. This stone is used in conjunction with Gold test acids to determine if solid gold and karats Test stones are used to test your gold jewelry without ruining it, by using the stone to test instead of directly on the piece.