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.