I just received one of these in the mail. It's a 10x2.5x1 cm double sided whetstone intended for knife sharpening. It has two sides with two different grit: 5000 and 12000. The green side is made of corundum which is supposedly the next hardest material after diamond. I got it off ebay for about $7 and I have to say that I've already adjusted nibs on 3 different pens (including my brand new Wing Sung 618) and the results are very positive. I started with light, 8-shaped strokes on the 5000 side and then did the very fine adjustment on the 12000 side. My Wing Sung went from sharp nail to butter soft in a matter of minutes. I am very pleased with this item and I feel that it's going to be all I am going to need in my nib grinding tool set for quite some time. Have you ever tried this?