I may have mentioned this before but a couple of years back we were having a heck of a time getting a mirror polish on an obsidian cab in my class at William Holland. We were using diamond on moleskin up to 100,000 and the stone just would not pop. I remembered what an old rock guy told me in Quartzite. He said to use a rough leather pad with super cerium oxide. I had some shipped in and the next day my student had a fantastic polish.