comments:
If you can get your sanding up to a pre-polish of about 1500 or 2000 grit, then get some felt tips for your Dremel and using cerium oxide would be a "normal" solution for a lot of the typical Mohs 6 and 7 materials out there. I suggest you get the guru Sinkankas' book "Gem Cutting: A Lapidary's Manual" which will give you a very good feel for the basics and advanced ideas on how to work on rocks. To see how to use the felt tips with the Cerium, check out Roy's Rocks' YouTube channel where he uses that method to carve and polish opals. Many tips and tricks there. https://www.amazon.com/%F0%9D%90%8B%F0%9D%90%AE%F0%9D%90%A8-Polishing-Compressed-Dremel-Rotary/dp/B093DW29PV Those are the sort of tips I am talking about. I cannot give any advice on tumbling, as we do not do that, but I do get the impression that the grits and final polish differs a bit.