What is Precision Gem Cutting?

Time-consuming!

To me a precision cut gem has, in order of increasing importance:-

• accurate cutting – facets meet where they should.
• a cut optimised for light return / sparkle for the gemstone material rather than an inferior cut intended to minimise losses of material during cutting.
• well polished facets - especially on the pavilion (the bottom of the stone).

Sapphire as an example

Lets say I am cutting a sapphire to approx 10mm final size.

Typically a gem of this size will have approximately 60 individual facets. I will typically rough out the stone using a copper lap surfaced with 25 micron diamond abrasive. This cuts fast though leaves damage below the surface. Then recut all the facets at 3 micron using a zinc-tin alloy lap. This is slow but allows precise cutting and produces to the eye a well polished surface. Many commercial stones are polished at 3 micron. A 3 micron polish looks OK to the naked eye but markedly reduces the brilliance of the stone – the trick is to further polish the surface such that light waves see no imperfections.

The facets are repolished on a ceramic (essentially sapphire) lap using 0.25 micron diamond. This is smaller than the wavelength of light at 0.35-0.75 microns. Visible light waves do not ‘see’ these very fine scratches – diffracting around them – a physicist would refer to the Rayleigh limit here. This can be relatively quick or horribly time-consuming - usually the latter. I prefer ceramic laps for final polishing of sapphire, being very hard they do produce the flattest facets. Ceramics can be tempremental - they are respected rather than loved by many faceters.

In summary, assuming nothing has gone wrong – which is rare – we have cut / polished 60 facets to a precision of better than 1/100 mm or approx 1/100 of the degree of slope. Each facet has been cut on three laps – being examined each time. That is 180 steps.

© 2021 Ian Robinson email: Ian