Trugrit also has 2x72 silicon carbide belts to 15 micron which is 1200 grit, $4.95 ea. They also have no grit 2x72 cork belts you can use to put diamond compound into. Use an oil based compound and it won't come out using water if my experience with leather is any experience. Those belts are 6.50 ea. Diamond compounds are not that expensive when you consider how long they last. I use Boride Engineered Abrasives for mine, there are many others. I like Boride as their quality is high and customer service is great. Try Ebersolesrocks on Ebay too for the compound and Amazon sells it too, but I had some problems with DiamondTech sending me the right stuff. You want medium to heavy concentration too. You do NOT want light. (10% concentration). I do not think you will find diamond belts much cheaper if you can find them at all btw. 9 micron is 1800 grit by the way and therefore the scratch lines will be barely visible held in the right light..
The diamond compounds are not exactly like grit, for instance my 500 grit seems more like 800 grit sandpaper. At least on hardened steel. What gemstones are you polishing? Some like turquoise or beryls like emeralds are not very hard. I use compounds to 4 microns which is 8000 grit. It lends itself to being worked into leather very well so I would think it would work well in cork too. A 2x72 belt would take up at least 3 grams or more. I use Tap Magic cutting oil to thin it and work it into the leather, then the Tap Magic mostly evaporates. I also use medium hard felt buffs, if you have a variable speed buffer you may want to consider that route, but as the buff loads up it will need to be cleaned. I don't think you will need water to just polish the gemstone, to shape it yes, but polish I don't think it would be necessary. Boride has water based too, but I do not know how long it would last with water dripping on it.
Shape it and take it to 15 microns with the sc belt and then polish with diamond, I polished out some scratches in my daughter's opal by hand using 500 to 1200 to 5000 grits on leather glued to small boards or sticks if you will. Took me 15 minutes, if that.
Here is a grit-mesh-micron conversion table.
http://www.gessweincanada.com/category-s/11328.htm