Professional Jeweler Archive: Rough Report

March 2002


Rough Report

Rough diamonds and their indicator minerals conceal clues that more and more diamond traders are keen to know about

Gemologists are generally trained to identify and understand cut diamonds – they appreciate rough diamonds for little more than their aesthetic characteristics. Elena Semenets, a mineralogist at European Gemological Laboratory, sees rough diamonds differently. She detects clues in the crystallographic features that provide an indication of the diamond’s origin. Sometimes she spots evidence in the crystal’s shape that suggests the diamond’s type. These observations, among others, are increasingly important to those who deal in diamond rough.

Semenets and Branko Deljanin, EGL-USA’s chief gemologist, have put together an educational program on rough diamond grading for the trade. So far they have been invited by Canadian mining and polishing factories to give the seminars; those begin later this year, coordinated by the Gemmological Association and Gem Testing Laboratory of Great Britain. Semenets and Deljanin also presented a lecture about rough diamond analysis at the Tucson gem and mineral show in February. The lecture was sponsored by the American Gemmological Association.

Semenets’ expertise is based on the morphology of diamonds of Russian mineralogist Orlov, whose system helps mineralogists type diamonds according to their external features. “The way a diamond looks on the outside often reflects what is happening on the inside,” Semenets says. She is also translating Morphology of Diamond Indicator Minerals by Valentin Afanasiev from Russian to English. It will be published later this year. Now conducting mineralogical and rough diamond research for EGL-USA, she says demand for information about rough diamonds is escalating.

In addition, EGL-USA is studying kimberlite samples, visiting diamond mines and performing thin sections on indicator minerals to help its customers determine the viability of a particular diamond-bearing kimberlite pipe. As a result of these new demands, the laboratory says it has been issuing Rough Diamond Analysis Reports since 2000. The reports are based on rough diamond observation and include such information as the diamond’s weight, estimated recovery weight, clarity, morphology, surface color and fluorescence, recommended price per carat, recommended cut shape and measurements. Different segments of the trade have found uses for the Rough Diamond Analysis Reports, including determining origin, type and whether the gems have naturally or artificially irradiated.

Determination of Origin

Opinions about rough origin are important to rough diamond producers and buyers – they need information about origin because of conflict diamond issues or want further advice about the average quality and value of a mine run. The report provides opinions about a diamond’s origin in the comments section, if and when such a determination can be supported by additional scientific evidence. This service has stimulated interest from prospectors and miners in Brazil, Africa and Canada.

Determination of Type

The rough diamond reports have interested an increasing segment of the diamond trade – those looking to buy rough diamonds based on their type (which is based on the relative presence of nitrogen impurities incorporated into its crystal structure). Interest in buying diamonds by type has soared as high-pressure/high-temperature techniques become popular. (Type IIa diamonds, for example, respond positively to HPHT processes by turning brownish diamonds to colorless).

Determination of Natural or Artificial Irradiation

Natural or artificial radiation is easier to determine when a diamond is in the rough stage. “There are some things you can tell by looking at a diamond’s skin; certain clues based on radiation spots and morphology,” Semenets says.

Determination of Cut/Color

Those who buy only small amounts of rough often need advice about certain pieces, such as the preferred shape a cut stone might be depending on the dimensions of the rough.

In its reports, EGL-USA recommends the final shape for a given piece of rough and estimates recovery weight percentages and final weight of a cut stone. It also determines what the finished color of the diamond will be.

– by Robert Weldon, G.G.

EGL-USA rough diamond expert Elena Semenets says there are visual clues people dealing in rough diamonds can train themselves to recognize. A sampler:

Type IIa diamonds don’t have recognized crystallographic forms and are often seen as cleavage portions or flats.
These are generally very stressed crystals. In a polariscope, they often exhibit interference colors and show “Tatami” effect patterns. (Tatami patterns look like carpet fibers).
Type IIa diamonds often exhibit a typical “hillock structure” in their morphology, as in the rough at left.
Greenish diamonds, like the rough on the left of the photo, have relatively young radiation stains that occurred near the surface, probably from uranium or another metal source. The stains in these diamonds are generally shallow and may be lost at the cutting wheel. The deep etching in this diamond’s skin can be attributed to warm climates (in this case Central Africa). Kimberlitic diamonds come from deep within the earth’s surface. They are transported by alluvial forces. Those like the rough on the right, redeposited under sedimentary rocks at great depths and temperatures over 500° C, exhibit brown radiation stains in a microscopic honeycomb pattern.

Diamonds are courtesy of EGL-USA; Photos by Robert Weldon.

Copyright © 2002 by Bond Communications