CAM

The advantages of a shallow preform for some designs.

Rough saved but perhaps as import, not needed.

With modern meetpoint designs the Centrepoint Angle Method, aka CAM, for establishing gem outlines is arguably the most popular currently in use. For many common designs this involves cutting the pavilion break facets to a temporary centre point and then cutting girdle facets at the same indices. For designs like the Standard Round Brilliant this is highly satisfactory and involves only a small reduction in weight when the mains are cut in. However, for designs with steep break facets and substantially shallower angled lower pavilion facet tiers this method can result in very poor yield and / or the conclusion that a particular pieced of rough, while wide enough, is not deep enough to provide a stone of the desired width.  Here’s an example.

Christmas 2024 saw a request from a granddaughter for a flashy pendant. This involved a stone in a particular colour of CZ, and with a diameter of about 10mm of which I only had one piece of rough. This was a sawn block measuring 11 x 11 x 7 mm, and below is the design she liked based on the animated rendition available in GCS.

Freya's Xmas Sparkler

Here is the design, modified as described below: -

The ‘traditional’ way of cutting this would be to cut the pavilion break facets first. That is tier 2 in the diagram above. However, when I looked at the depth of rough required to produce a 10mm stone using this method it was evident that the piece of rough I had was nowhere near deep enough. In fact, with the depth of rough I had, the maximum diameter I could achieve was 7.65 mm. See the lower diagram below. Nevertheless, a solution was available. This involved a shallow CAM preform and resequencing the pavilion cutting order. None of the faceters I have mentored have known about this method and while it has been described by others, e.g. Fred van Sant and Tom Herbst, it appears to be a little known but highly valuable technique. Using this, I was able to produce the desired 10 mm stone from the 7mm deep block of rough using the cutting instructions above, and allowing for a little wastage based on a slightly less than perfectly even cube of rough, as shown below.

There are two ways of looking at this in terms yield. Based on the diagrams above, the shallow CAM preform method yields a 10 mm stone weighing 6.64 ct in CZ. Cutting it the ‘traditional’ way yields a stone of just 2.89 ct. Should rough of the same width but of sufficient height be available to cut a 10mm stone using the ‘traditional’ method, the weight of that rough would be c. 30% greater than that required to cut the stone using the shallow CAM preform method. Not a big financial consideration when cutting synthetic materials and other low cost rough but a major consideration when cutting expensive rough.

As a closing remark, CAM preforms, shallow or otherwise, can also simplify the establishment of an accurate outline for many published designs. These include some of those based on the potentially inaccurate Equal Centre to Edge Distance method.  This relies on an accurate machine with minimal flexure, very careful hand pressure and lots of experience. Maybe a blog post to demonstrate this in due course.