I dont have many tips but this one is red hot.

In Australia, where the Opal comes from, the mud it is found in is of the smallest particle size yoou will find anywhere. It is a lot smaller than silica so less than .05 microns is my guess. This is finer than cerium oxide, aluminium oxide etc that is used in polishing.

You can get a lump of this mud from anyone that cuts Opals, it is in the bottom of their cutting tray and a pest,

Here is a tiny speck, smallest I could drop, then I blew on it and half went up in the air and you can see how much covers and this is not as fine as it gets coz I scraped it off with my fingernail. Its very good for cheap.



Pics here http://gallery3.kitmaker.net/showgallery.php/ppuser/73230/cat/500