It is a single use disposable item, that will not even work! I needed to deepen existing nut slots on my ukulele for about 0.75 mm. I spent more than 4 hours filing! And I only managed to deepen ~0.5 mm before the files became as smooth as oiled baby skin. Spent more then one hour for each nut slot! And used up 7 or 8 files. I have read all the reviews and was expecting them to be flimsy. And yes they were very soft and bendy. But I was expecting them to be harder then the GraphTech Nubone XB nut, otherwise they serve no purpose. My nut is white, but because all the metal from the file is shredding off the slots became black and you can not completely clean it off after filing. Not sure how it i will affect strings from all the metal dust inside.
Also the stone block is neither straight not flat, it is just useless!