Anyone who re-polishes worn stones, or who tries to remove a scratch from a table facet, will be familiar with a common problem. Some stones, in my experience particularly tourmaline, appear to develop a resistant ‘skin’ during polishing, which impedes the re-polishing process. The effect is that you cannot re-polish the facet, which just slides over the lap, with your usual polishing combination. I think it is due to work-hardening of a surface layer; but there are other opinions about what causes this. The solution is to grind away the polished surface. A 1200 mesh lap often cannot penetrate the hardened surface, and you have to resort to a 600 mesh lap. At first nothing happens and then the surface starts to break up unevenly, as in the photograph. When all the previously polished layer has been removed a very rough subsurface is exposed. This has to be removed completely to expose undamaged material

before trying to re-polish the facet. For this pre-polish step the 1200 mesh lap may be adequate on softer stones like tourmaline, although a pre-polish with 3000 mesh or 8000 mesh diamond is advisable before a final polish. Knowing that you are likely to be confronted by this hardened layer means you can plan the re-polishing of the stone taking it into account. Re-polishing a table facet often results in the need to re-polish all the star facets and sometimes the entire crown of the stone because of the lowering of the table. 

Duncan Miller