By far, the best combination for polishing garnets and tourmalines is a phenolic lap and an aluminium oxide (Al2O3) abrasive. I’ve often used and recommended tin laps and oxide polishes for these stones, but tin laps are just too expensive. I’ve found that phenolic laps work just as well or better over time. (I’ve included some tips for using tin laps below).
Applying Polish To The Lap
Use a high-grade aluminum oxide polish, about 1 tablespoon polish to 2-3 tablespoons of water, in a slurry. Add a small amount of vinegar to help stop scratches, though that’s seldom a problem. Apply the polish to the lap with a decent-quality watercolor paintbrush made for applying washes. It will hold a good amount of liquid/polish.
Run the lap just fast enough that it doesn’t throw off the polish and keep the gemstone moving in a nice, sweeping arc.
Using A Tin Lap For Polishing Garnets And Tourmalines
In my opinion, a tin lap works better if it’s scored. The scores help keep the polish on the lap longer. This speeds up polishing and also seems to help remove waste from the polishing process. If you haven’t used a scored tin lap before, it may feel a little strange at first. However, I think it does make a difference.
Of course, you can polish without scoring, too.
Notes On Polishing Garnets And Tourmalines
I cut and polish very often and prefer having two separate polish laps, one for garnets and one for tourmalines. Garnets are slightly harder than tourmalines. On rare occasions, I’ve had a garnet chip in a lap scratch a tourmaline.