Calibrated Gemstones

Most of the time, when we’re working with materials worthy of custom-cutting, we aren’t going to be producing calibrated gemstones – chopping the rough down to a specified size – because we’re working to preserve every tiny bit of the gem.

In those instances where we’re tasked by a jeweler with producing a stone to fit an existing setting – or if we’re producing a stone to fit a standard commercial setting – it’s important to have the skill of calibrating.

Although we cover calibrated gemstones (and how to calibrate them) in the live academy, faceting designs that show a step titled “calibrate length” or “calibrate width” can still be puzzling for some students.

All this means is if you intend to calibrate the stone (make it a specific size), the step with that instruction is the step where that is to be done.

For instance, in my 85-facet design “Easy Omni Oval”, the first step of cutting the pavilion is to cut 16 facets, on various indexes, to a point at 44 degrees. When that step is complete the gem will look like this (GemCAD):

calibrated faceted gemstones

The very next step says “calibrate length”. It is a set of girdle facets cut on the 21 index, which leaves the gem looking like this (GemCAD).

calibrated faceted gemstones

So, we’ve cut the END facets of the oval pattern, dictating the LENGTH of the finished oval. If we want that oval to be a specific size, we can calibrate (measure) the length – adjusting mast height and trimming more as necessary – to produce the exact length dimension desired.

My “Easy Omni Oval” faceting design has a L/W ratio of 1.237. So, if we calibrate the length to 10mm, the width should complete at about 8.08mm – which is well within the tolerances for a commercial 10x8mm oval setting.

calibrated faceted gemsSo, wherever a design gives the “calibrate length (or width)” step, you can usually change that dimension at that step without affecting the remainder of the design. It’s your opportunity to adjust in that way, because afterward changing the dimension will no longer be trivial. This should make it easier to cut calibrated gemstones whenever the job requires it. Happy faceting!

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1 Response to Calibrated Gemstones

  1. David says:

    Great post, John. Thanks so much for the clarification!

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