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Begin with the End in Mind: Tips, Tools to Evaluate Diamond Rough


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Chandra Horn, academic director for Canada’s Montreal School of Gemmology, said that when evaluating diamond rough, “You’re always imagining, ‘What’s going to come out of this? What’s in there?’” Photo by Kevin Schumacher/GIA
Equipped with a 10× jeweler’s loupe, a millimeter measuring gauge and a standard calculator, artisanal miners around the world have everything they need to determine realistic – and profitable – values of the diamond rough they mine by hand. A copy of the Rapaport Diamond Report wouldn’t hurt, either, said Chandra Horn, associate director of the Ecole de Gemmologie de Montreal, in an October 2015 lecture to students at GIA’s Carlsbad campus.  

Horn said the challenge for artisanal miners – who make up a significant percentage of her school’s student population – is a lack of access to evaluation software used by large-scale manufacturing facilities or organizations, such as DTC/De Beers. They also have fewer samples of rough to compare their finds with to help them determine what manufacturing and other costs will be.

The key, Horn said, is to begin with the end in mind – considering the final product through each step of the evaluation.

“The whole goal of these gems is to end up as a finished product,” said Horn, who also teaches rough diamond evaluation. “When we start looking at how to value and grade these diamonds, it’s almost as if we have to pull out a crystal ball and look into their future and try and predict what they will become, in order for us to work backwards to see what value they have in their current state.”

Horn presented the “rough C’s,” or evaluation criteria, which she described as comparable to GIA’s 4Cs of diamond quality for polished stones:

4Cs for Polished Diamonds vs. Evaluation Factors for Rough Diamonds
Polished Rough
Carat Weight Carat Weight
Cut Shape/Make (sometimes Model)
Clarity Quality
Color Color
With knowledge of those criteria, as well as the calculator, loupe and gauge, Horn said her students – and others without access to large-scale manufacturing operations – can effectively evaluate rough.

Horn said DTC/DeBeers sorts diamond rough into 11,344 categories. While artisanal miners don’t use that many, a few simple tools give them everything they need to determine realistic, profitable values for the diamond rough they mine by hand. Photo by Kevin Schumacher/GIA

First Steps: Size and Shape

The ideal first step, she said, is to sort a parcel of rough by size or carat weight. Miners and manufacturers must consider both how the rough (most frequently shaped as cubes, octahedrons or dodecahedrons) can be cut – considering the external and internal − and which polished shapes have the highest market values.

 “We are always looking to identify the shape so we know the orientation inside,” she said.

“Cutters have to pay attention to and respect the internal grain, whether that is the cleavage direction or differential hardness we see in diamonds. These are going to dictate what it is we can do with these stones.”

When a stone’s internal directions aren’t obvious, Horn said that unique surface features can provide clues. “The tiniest little octahedron is going to tell the cutter where they are and what direction they’re looking at.”

Horn reiterated that the rough’s final shape plays a huge role in the process, too. Most of the time, particularly when people are not evaluating for a large manufacturing facility, round brilliants will be the most desirable final shape.

“Why? Because rounds are what sell,” Horn said. “Rounds are what we see in the marketplace, and rounds are not going to sit in your safe.”  

She acknowledged, though, that there was a great opportunity for profit if the next up-and-coming jewelry designer in the student lecture hall could figure out how to promote triangle-shaped diamond engagement rings.

“A triangle-shaped engagement ring trend could change the value of the macle (triangular-shaped rough),” she said. “If you can create a market for them, there are lots of these stones out there. A lot of rough dealers have these stones and it’s not worth it to them to cut them as triangles.”  

“When you work with rough, you’re always looking for the outline of your stone,” Horn said. Photo by Kevin Schumacher/GIA

Rough Quality: Inclusions

Diamond rough’s quality – the equivalent to polished goods’ clarity – is the next aspect for evaluation. Inclusions and their reflections, as well as areas of stress and strain, must be considered.

“Inclusions can change what you can do with a stone,” Horn said. “Not only do you have to have an understanding of a little bit of crystallography and a little bit of what the cutter or manufacturer is going to do, but you definitely have to understand how the diamond grader is going to look at this finished product. What effect is that going to have on how this is graded as a polished stone?”

Showing the audience examples on the screen, Horn demonstrated how inclusions can be surrounded by areas of stress or strain.

“Pockets of stress areas and strain can lead to the stone literally exploding and/or shattering while we are trying to saw it, polish it or place a facet on it,” Horn said. “Usually, (it’s the cutter’s job) to avoid this, but if you’re buying or valuing or grading rough diamonds, you have to be conscious of it, too. Because it just changed what we can do with this stone … it is no longer simple. We have to factor this in as a danger zone.”

The sawing process itself can present risks to an otherwise “clean” stone: Horn displayed a close-up image of a piece of rough from her own collection, with a feather that hadn’t existed prior to sawing.

“They were going to cut it, do an uneven split, and get a nice, round, large stone out of this,” she said. “And then boom!” The sawer was working along the correct saw plane, but “sped it up too much – was in a bit of a rush – and provoked this feather.”

“Meticulous measuring and math – they’re not what I signed up for, but what I have fallen in love with,” said Horn. Photo by Kevin Schumacher/GIA

Rough Color: Strike a Balance

Horn explained that when it comes to color, rough evaluation doesn’t utilize the whole alphabet. “There is no D or E,” she said. “It’s light, medium and color.”

In general, the “less is more” rule applies – except in the case of fancy colors, which are an entirely different proposition.

“The color’s not going to look the same once we get it cut,” she said, noting that people have to consider where the color concentration is and what balance it will strike in the polished stone. “With fancy-colored stones, you definitely want to be sure they are saturated colors.”

Horn said that in green diamonds, for example, which often have very thin stains that only penetrate the stone shallowly, the wrong cutter can take off too much “rough skin” and end up with a colorless diamond.  

Pinks and browns, which are often seen in color zones, are also at risk in the hands of a cutter unskilled with colored diamonds.

“That zone – that little area that held that pink color – which, as you probably know, with some of the auction results in the last few years, adds a lot of zeroes to your value – you don’t want him to polish that off,” she said. “That would be a big mistake even if it came out as a D-color (polished stone).”

Horn said that overall, “a lot of factors” must be considered when evaluating rough, and “it’s all based on the prices for polished goods.”

“At the end of the day, it’s always going to come down to what kind of value will I get? What’s my yield going to be? What percentage of this rough am I going to be retaining in the cut product?” Horn said. “It’s always going to come back to math and finance.”

Jaime Kautsky, a contributing writer, is a GIA Diamonds Graduate and GIA Accredited Jewelry Professional and was an associate editor of The Loupe magazine.