If I understand correctly, there is no way here to pumice, but Siliglass. After a long search on the Internet, it seems that a combination of silicone and Siliglass fiber is (please correct if wrong). So not exactly a natural product. Unlike the pumice.
Nevertheless, I find the Horthautentferner - order the thing to refer once correctly - class.
It serves its purpose well. The coarse side is very pleasant and not so rough as some lava rock. The finer side smoothes wonderful to conclude the skin.
The area is sufficiently large. As unnecessary I feel the cord. I would prefer the Callus without hole.
An asterisk deduction because he is not very good in the hand. An oval shape or an enclosing rubber would make this grip and pleasant to the touch.
