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Gemstones

Articles from GEMSTONES - LOOSE (254 Articles)











Coloured stones cuts
Coloured stones cuts

Introducing coloured stone cuts

Coloured gems require a different approach to cutting than diamonds. In the first of a series of articles, KATHERINE KOVACS FGAA and TERRY COLDHAM FGAA detail the subtleties of the coloured cuts.

Comparing diamond cuts with coloured stone cuts is rather like comparing champagne (or “sparkling”, according to its new name) and red wine. Sparkling wine provides a light, crisp taste that complements its primary difference of bubbles; red wine is about subtlety, the complexities of a greater number of varieties and depth of flavour.

Like sparkling wine, diamonds are heavily marketed towards women. Coloured stones, like red wine, tend to be more of a connoisseur’s choice, or perhaps a taste that develops over time as the consumer gets to know more about each variety or type. Selecting one as “better” than the other is impossible, and strictly a case of consumer preference.

So too it is when trying to compare what is desirable in a diamond, as opposed to a coloured stone, when it comes to cut. Diamonds, with their high dispersion, are at their best when the maximum amount of brilliance and light is reflected out of the stone.

This is not always the case with a coloured stone – brilliance is important, but colour or depth of colour always reigns supreme.

That’s why it’s unlikely one may have seen too many cabochon-cut diamonds, or why one may choose a stunning, deep-red Burma ruby over a slightly-purple Thai ruby.

Diamonds are from the cubic system of crystallography, which in basic terms means that they can only display one colour.

Disregarding a few exceptions, such as spinel, the majority of coloured stones don’t belong to this crystal system, meaning that they may display two or more colours in varying degrees of subtlety and intensity, depending on the orientation of the stone when it’s cut. The job of the cutter is to emphasise the best, or most marketable aspect, of each coloured stone’s colour.

Coloured stones belong to a variety of different crystal systems, so different rules may apply depending on the type of stone that being cut. Further, the shape and size of typical rough crystals varies widely according to the family of gemstone and origin.

It is common knowledge that it is much harder to find a black piece of rough Australian sapphire than a bright blue piece of Kashmir sapphire; or that one is more likely to get a five-carat round topaz than a stone of same weight and shape in a Columbian emerald.

Understanding how different varieties of rough, coloured gems are usually cut can help a retailer when faced with a customer requesting something that isn’t in stock or is difficult to source.

The other common problem associated with using cut as a tool to sell coloured gems, as opposed to diamonds, comes from the differences in the size-to-weight ratios that each stone presents.

A one-carat coloured gemstone is unlikely to have the same proportions as a one-carat diamond. Sure, a retailer may be able to get a 6mm diameter stone in ruby, tourmaline, opal, aquamarine, or amethyst – it just may not be one carat in weight.

This is due to each gem variety having a different specific gravity – a gemmological property where different stones have a different “density” or weight, even though they may appear the same size.

A common bug-bear of coloured gem dealers is being asked for a “one-carat such and such”, and offering a stone for sale of similar proportions of a one-carat diamond only to be told that the stone must match the weight and not the proportions. For example, a stone measuring roughly 6mm but weighing say 20 per cent more or less than one-carat.

Depending on the variety, some gemstones may have to be cut “deeper”, or with heavier pavilion facets, in order for the gem to display a good colour. Pictured is a Columbian emerald (1) alongside a Sri Lankan pink sapphire (2), where the difference in millimetre-size is less than one tenth of a millimetre; however, the weight varies by almost a carat.

Other gems may have to be cut shallow; for example, a carbuncle cabochon-cut garnet where the bottom of the stone is slightly “hollowed” to lighten the colour.

When it comes to cut, the first step to selling a coloured gem is to understand that the rules for diamonds don’t necessarily apply. From here, this column will be looking at how different cuts are applied to different gem types and desirable qualities.



















Thursday, 13 December, 2018 07:27pm
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