By Duncan Miller

I started faceting in pre-GemCad days and found cutting ovals very laborious. I would cut the girdles by eye, using various oval templates, and placed the brilliant-style facets by eye too. Producing matching pairs was very trying. The advent of meetpoint faceting and GemCad overcame all these difficulties. Now there are lots of designs for ovals that are meetpoint, requiring no preform, with the girdle outline evolving out of the cutting sequence. You can access some of these at http://www.facetdiagrams.org/. Click on Search; select Oval on the dropdown menu Shape?; check the box Only show Open designs (with GemCad ASC files and cutting instructions); and click on I agree. Search! There you are – a huge selection of oval cutting recipies in every conceivable proportion. Many of the Long & Steele ones have interchangeable pavilions and crowns to give you overwhelming choice. (Of course, there are other internet sites with oval diagrams. One of my favourites is at https://www.gemologyproject.com/wiki/index.php?title=Faceting_Designs.)

Some of the designs on facetdiagrams.org do require preforms and some of those are not supplied. What to do? If you have GemCad you can open the file, download it (the blue download option, lower left), and open it with GemCad. Then you can apply the preform generating sequence described in the MinChat of October 2018 to produce the appropriate preform.

Many of the Long & Steele designs are for ovals with common length/width proportions. For these ovals Long & Steele provided a very convenient table to generate the required preforms, published in their book ‘Facet Design Volume 1 Ovals’, part of which I will reproduce here. This enables you to cut a suitable preform for a Long & Steele oval of specified L/W ratio without having to construct the preform yourself in GemCad.

 Extract of Table D-5 SIXTEEN FACET GIRDLE OUTLINES FOR SELECTED OVALS (from Long & Steele, Facet Design Volume 1 Ovals, second edition)

L/W ratio

Indicies (96 wheel)

Angle (degrees)

Typical size (mm)

1.20

02, 08, 13, 21, etc.

45.0; 43.5; 42.1; 40.1

5×6, 10×12, 15×18

02, 08, 13, 21, etc.

02, 08, 13, 21, etc.

45.0; 43.4; 41.8; 39.6

9×11, 18×22, 27×33

1.25

02, 08, 13, 21, etc.

45.0; 43.1; 41.4; 39.0

4×5, 8×10, 12×15, 16×20

1.29

02, 07, 12, 20, etc.

45.0; 43.3; 41.3; 38.6

7×9, 14×18, 21×27

1.33

02, 07, 12, 20, etc.

45.0; 43.0; 40.6; 37.6

3×4, 6×8, 9×12, 12×16

1.40

02, 07, 12, 20, etc.

45.0; 42.6; 39.7; 36.2

5×7, 10×14, 15×21

1.50

02, 06, 11, 19, etc.

45.0; 42.7; 39.2; 34.9

2×3, 4×6, 6×9, 8×12

Recently I have been learning how to cut ‘classical’ mixed-cut ovals without a diagram, receiving lots of help from ‘1bwana1’ on GemologyOnline.

(See https://www.gemologyonline.com/Forum/phpBB2/viewtopic.php?f=8&t=2501&start=30).

After several attempts I managed to cut a symmetrical oval pavilion with three tiers of lozenge-shaped facets, and satisfied my curiosity about how it is done. I think it needs a lot of patient practising, and I found guessing which angles and indices to use unsatisfying. So I set about creating a GemCad design to produce something similar.

Here it is, in a 1:1.30 L/W ratio, with a crown derived from Long & Steele:


https://www.gia.edu/gia-news-research/value-factors-design-cut-quality-colored-gemstone-value-factors

This link takes you to a Gemological Institute of America page with the first of five articles on coloured gemstone value assessment. The complete set of articles can be downloaded in one go from the first red DOWNLOAD button on the right hand side of the page. They are very informative and well-illustrated articles that anyone interested in faceting or the valuation of facetted stones could read with benefit.