Minecraft wiki entry for Grindstone, associated text: Placing two items (enchanted or not) of the same type in the input slots forms a new non-enchanted item of that type and with a durability equal to the sum of the durabilities of the two input items, plus 5% of the maximum durability of that item (rounded down), up to its maximum durability. Both input items are consumed in the process. If either input item was enchanted, then removing the output item drops some experience.