I haven’t had the issue until recently and I haven’t been able to place what the problem is, though I’ve seen it a few times and it usually resolves itself after a while of polishing.
When I polish anything softer than a synthetic CZ or corundum, I end up with rainbow streaks on the facet that are a pain to remove. Usually I manage a perfect polish on the harder stones.
For reference, I’m using a tin lap with 14k diamond powder and I tend to run the lap at medium speed (the 40-50 setting on a Fac-ette). I’ve tried lubricating with water, WD-40 and blade oil to see if it changed anything (even running the lap dry sometimes), but no dice. Polishing a harder stones works just fine without that strange streaking.
I’ve also tried on a ceramic lap with 50k diamond powder, though I don’t tend to use oil lubricants on that lap with the same problem. Anyone know what might be causing it? They’re not really scratches, but instead odd rainbow streaking that’s easily visible when tilting the facet in the light.
first you should have used the 50k diamond on the tin first and only. your machine has variable speed, we run our machines VERY slow and a water drip rate at 25-35 seconds per drip. slow speed and little water using only diamond powder applied by finger tip we create a muddy slur that does the work. too fast and too much water you sling off the polishing agent and water and create instant dry spots that can scratch. as for ceramics the surface is too smooth and the diamond or any polishing agent can’t sink in and do its abrasive action, the diamond just roles like a bearing. as for compounds and oils–no– they are lubs not wetting agents and don’t compress enough to get good action from you diamond. we polish everything on our tin laps, all hardness’s with no problems.