By Duncan Miller

This was another jeweller’s request. The setter had broken one of a matching pair of blue-green stones, destined for earrings, bought by the client in India as emeralds. They were apatite; but nevertheless the broken stone had to be replaced to fit the already-made setting. Fortunately I had just one piece of blue-green apatite that matched the colour. In order to produce a stone of the same size and proportion I had to replicate the oval precisely. I could have slapped facets on it and hoped for the best, but being a precision cutter I made a diagram to guide me. To produce the correct girdle outline it required a preform, for which the rough was too shallow to cut to a point, so I built it up with low temperature green wax. Once the girdle was established, following the cutting pattern was relatively easy, although not the sort of thing a meetpoint faceter would relish doing.

I am going to use this example to demonstrate how to us Robert Strickland’s GemCad program to make a preform from any faceting diagram that may need one. First, you need a copy of GemCad, which you can download for the very modest fee of US$95 from Next, you need a GemCad copy of the design for which you wish to make a preform. This presumes it doesn’t have a girdle preform already and isn’t a meetpoint design that generates the girdle outline as you cut the design (like the Omni oval posted last month). Then, in GemCad you delete all the facet tiers except for the girdle facets. This leaves you with a tall prism shape. Simply cut a series of facets at the girdle indexes to meet at a common point on top. There, you have your preform. What angles to use? I usually start with 40˚ for the steepest facets, the ones where the girdle is nearest the centre point, but once you have the preform you can tangent ratio the facets in GemCad to accommodate your rough and the design.

Next month I will describe how to produce preforms for ovals of any standard proportion from scratch.

Is the gemstone in the above picture aquamarine or glass?

Look for tell-tale air bubbles in the gem – very often, if glass, they are there.  JW