If you want to use the aluminum master lap for final polish, you can use a dynalap. They come in many different grits, the blue spectra lap is popular for that. The downside is that it can cause rounded edges on the facets.
your best bet is to find an affordable clean tin surface lap. use 50,000 grit diamond powder– no oils– just clean water at a VERY SLOW drip and lap speed very slow too. oil is a lubricant, use water which is a wetting agent and try to create a muddy slur. the diamond powder will charge into the tin lap as being used which prolongs the polishing, you’ll generally feel a small drag action when polishing. i have a 13 yr old girl student that it doing GREAT and great polish on all her different materials stones. she’s just been written up in the september 2019 issue of rocks and gems magazine. she is a junior in the lakeside gem and mineral club of kennewick wa.. i’ve tried zink, lead and other metal laps for polishing and unless your cutting a diamond you can’t beat a good tin lap and 50,000 diamond powder.