In the bad old days, one cut facets on a 600 mesh lap, followed by a 1200 mesh lap and then went on to polish. The 1200 mesh leaves quite deep scratches, and on some material produces ‘orange peel’, a mottled surface with alternating rough and smooth patches. This makes polishing tedious. A pre-polishing step, with 3000 mesh or 8000 mesh diamond gets rid of the scratches and any orange peel. You might think the additional step adds time to the process, but in practice it speeds it up because it makes polishing so much quicker and easier. So, 3000 mesh or 8000 mesh, and on what kind of lap?

If you can afford it, you can buy a 3000 mesh sintered bronze diamond lap and pre-polish on that. I use diamond paste on a metal lap because it is a less expensive outlay and enables you to ‘tune’ the surface – more about that later. The metal lap can be old-fashioned tin/lead alloy, or a more recent lead-free tin alloy like Gearloose’s Batt laps, or traditional copper or high quality cast zinc. The softer laps allow the diamond particles to embed further, so they work better with 3000 mesh diamond. The harder laps, like copper and zinc, work well with the finer 8000 mesh diamond. Whether you use oil-based or water-based diamond paste is a matter of your choice. Either way, apply the diamond paste very sparingly. You can use a steel roller, a piece of synthetic corundum or even the flat face of a quartz crystal to embed the diamond, and then wipe off excess gunk before starting to pre-polish. If scratches appear on a facet there is either too much diamond on the lap or a build-up of swarf – black gunk that must be removed. This you can do with a piece of paper towel dampened with the appropriate lubricant for the type of diamond paste, wiping the spinning lap from the centre to the periphery. If necessary you then can apply another small amount of diamond paste to produce a quick pre-polish.

What about the ‘tuning’? Once you have removed all the scratches and any ‘orange peel’ you should have a nicely even, frosted surface. If now you clean off the lap with a piece of dampened paper towel only the embedded diamond particles remain. A few sweeps of the facet on the cleaned surface will produce an even finer pre-polish finish than before. For larger facets you can tune the lap by adding a bit more diamond paste to make it more aggressive. On smaller facets you can tune the lap by cleaning it to slow down the abrasive process. You can vary the behaviour to accommodate harder or softer facets on stones with marked differential hardness. Clean your stone and wash your hands thoroughly with warm water, soap and a scrubbing brush before moving to the polishing lap.