Best Index Numbers for Ovals

by Fred Van Sant

How does one arrive at the best set of girdle index numbers when improvising or designing an oval with a faceted girdle? The most eye-pleasing shape is that which is as close as we can get to the perfect ellipse, but without a special computer program the index numbers to get that shape are guesswork. The sets of index numbers given below were obtained from my MacGem program.

Since 16-sided and 12-sided ovals are the most popular, and the 96 Gear is most generally used, those combinations are given here. For each range of L/W ratios, read the corresponding set of index numbers to use.

For preforming your stone, I can’t give you a set of C.A.M. angles because they would vary within a given range of L/Ws.

For improvisers, if you use the first and lowest index number of the set for the sides of your stone, and then use the last and highest index number for the ends of the stone, you should be able to eyeball the in-between facet or facets to get a nice elliptical shape. Go very slowly with the between facets. Index numbers alone do not guarantee the shape. Slight overcutting or undercutting will result in corners sticking out noticeably, or sides that are too long or too short.

For designers, the depth of cut of the in-between facets must be determined, or a method of locating the girdle points so they lie close to the elliptical curve should be found.

FOR 96 GEAR – 12 Sided Oval
L/W Range Index Set
1.10–1.13 4-11-20
1.14– 4-11-19
1.15–1.21 3-11-19
1.22–1.39 3-10-19
1.40– 3-9-19
1.41–1.60 3-9-18
1.61–1.62 3-8-18
1.63–1.69 2-8-18
1.70–1.87 2-8-17
1.88–1.99 2-7-17
2.00–2.20 2-7-16
FOR 96 GEAR – 16 Sided Oval
L/W Range Index Set
1.10–1.17 3-8-14-21
1.18–1.20 3-8-14-20
1.21–1.22 2-8-14-20
1.23–1.25 2-8-13-20
1.26–1.40 2-7-13-20
1.41–1.47 2-7-12-20
1.48–1.52 2-6-12-20
1.53–1.59 2-6-12-19
1.60–1.77 2-6-11-19
1.78–1.82 2-5-11-19
1.83–1.89 2-5-10-19
1.90–2.O1 2-5-10-18
2.02–2.08 1-5-10-18
2.09–2.20 1-5-9-18

These sets are for *elliptical* ovals. There are other shapes called “oval” which are not elliptical, and if your primary purpose is to maximize carat weight, then you might choose bigger and uglier over smaller and more beautiful, and use any set of index numbers the shape of the rough indicates.

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