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.

To Stack or Unstack Mains?

Among gemstone designers – and cutters – there has been a long and ongoing debate concerning the relative merits of gems in which the crown main facets are aligned with the pavilion main facets (stacked mains) versus those in which the crown main facets are placed between the pavilion main facets (unstacked mains). Some argue that ‘unstacked mains’ designs produce more sparkle (scintillation) but in terms of some designs, including the Standard Round Brilliant, there appears to be little difference in performance. Furthermore, in this writer’s opinion the traditional stacked version of the SRB gives a somewhat crisper face up appearance than the unstacked version.

The Standard Round Brilliant with stacked mains, left, and with unstacked mains, right. Renditions from Gem Cut Studio.

However, for some designs there is no question that an unstacked version produces a better visual effect than the stacked version of the same design. Here is an example involving an 80 index design requested by a family member fascinated by all things to do with pentagrams. The design is called Binary Star (see a 2022 post) and involves the use of similarly proportioned main facets on both the crown and the pavilion, with those on the former being truncated by the table. Apart from incorporating the asked for pentagrams in physical form, my objective was to produce a design in which a five-pointed star was also an obvious visual feature. Before cutting the design in a natural material I decided to test cut both a stacked version in synthetic spinel with a RI of 1.72 and an unstacked version in synthetic corundum with a RI of 1.76. With suitable minor adjustments in GCS both produced similar renditions of both versions of the design with renditions of the unstacked version suggesting that this, rather than the stacked version, would produce the visual effect I was looking for.

Binary Star with stacked mains on the left and with unstacked mains, Binary Star (Decoupled) on the right.

Computer renditions of designs, particularly those from GCS, are a great aid but nothing beats physical observation of cut stones. Here, left, is the stacked version in spinel, and right the unstacked version in corundum.

Binary Star with stacked mains on the left and with unstacked mains, Binary Star (Decoupled) on the right.

In the case of this design, there is no doubt that the unstacked version produces a much better visual effect.

Designs for small sapphires

A lucky but very hardworking faceting friend in Australia has a couple of mines producing some remarkable sapphires. Much of the material recovered is of good size and clarity but inevitably some of it is only suitable for producing small finished stones. Many conventional designs have a large number of facets and are not viable for small stones. Here are some simple designs first published in the Australian Guild's magazine, Facet Talk. In their entirety these designs are unique but one crown was inspired by Andrew Brown and another by Arya Akhavan. Designs for other shapes to follow when published.