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  Looking to the future: The challenge of predicting color in the finished gem

By Guy Borenstein FGA EGG, Menahem Sevdermish FGA D.Litt., and EGC team
August 16, 2012


One of the most sought-after and hard-earned talents that one seeks from rough gem buyer is the ability to predict the eventual appearance of the as-yet non-existent cut and polished end product. Sometimes the rough mineral which is being offered for sale displays a distinct uneven color, spreading along the crystal, which looks unattractive to the untrained eye. But the experienced rough buyer sees its real potential.

When inspecting a rough stone, the buyer's imagination automatically begins planning the cut jewel within. By properly orienting the gem within the rough, the resultant faceted gem could cause the different color zones to merge. And, although some color zones could remain noticeable, the overall reflected appearance of the gem will improve dramatically, displaying an attractive color. But miscalculating the orientation could produce a disappointing result, one that is unattractive visually and painful financially.

This month we will demonstrate the intricacies of such a case. As it happens often, the European Gemological Center (EGC) team consulted with us about a pinkish orange-brown tanzanite rough crystal, given to them by a customer who wanted to try and produce a pink tanzanite polished stone.

We took several images of the crystal and analyzed each of its color areas separately, using our GemePro Sampler feature. As seen in Figure 1, the color zoning of the rough ranged from Medium dark, Grayish/Brownish, yellowish Orange (6-6/1) to Very dark, Grayish/Brownish, reddish Orange (4-8/1). The Sampler suggested that the resultant average color (6-7/1) of the fashioned gem would be at the orange-brown border area. No chances for pink tint were suggested.

We conveyed our analysis to the EGC team and the customer, stating that the end result is anything but pink.

Yet, the customer decided to try for the pinkish tint. He believed that that even gentle shifts could produce significant changes. Every slight change in the amount or in the position of one of the inner colors may dramatically influence the overall color of the fashioned gem. He planned the rough in a way that be believed would emphasize the pink and result in a pink or peach-like colored tanzanite gem. He even named it peach tanzanite

But we never promised him a rose garden, or more appropriately a pink rose garden. We told him it is orangey brown. After crossing our fingers on his behalf, and observing the cutting and polishing of the gem, our Sampler ws proven to be accurate. The overall color changed to a brownish orange (Dark, Grayish/Brownish, orange, or 5-7/1 according to the Sampler analysis). Not even the slightest pinkish tint was left.

The results were very close to what we had predicted. Examining it from all directions, we discovered that the orange-brown areas were close to the pavilion facets and therefore reflected on the entire gem, eliminating any pink traces. Another assumption was that some of the pink may have vanished during the polishing.

This was a good example about the accuracy of color prediction. Although planning the cut may sound as easy as 1-2-3, it is a very tricky discipline. Beside the color orientation, one must remember that other factors may apply, such as clarity flaws, cleavage directions, market demand for sizes/shapes, and more. In this specific instance the customer would have done well to have put his trust in our Sampler tool but even it cannot replace a serious gemological knowledge, years of cutting experience and a little bit of luck.

This time the customer was not lucky and a brownish orange tanzanite was produced. The customer did not have a name for it, or maybe he was at a loss for words.

If you have a magnificent gemstone or colored diamond and would like Gemewizard® to analyze it in one of its next Gem Color Reports, please contact us at info@gemewizard.com.

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