Gem Identification: Geology vs. Geography

Asapphire from Kashmir is worth more than one from Sri Lanka simply because of its locality. Exotic origins are part of the romance of gems. Throughout most of history there were only a few known gemstone deposits, making country of origin easy to identify. But as the number of gem localities continues to grow, determining country of origin is increasingly difficult.

Chris Smith, the Gemological Institute of America’s director of East Coast gem identification, says it’s now much more likely to find sapphires and rubies from different geographical localities with the same geological and mineralogical features.

Gemologists Dr. Daniel Nyfeler, of the Gübelin Gem Lab, and Dr. Henry Hänni, of the Swiss Gemological Institute, maintain that while identification is difficult, it’s still possible to identify country of origin.

In a three-part series of articles written for Jewellery News Asia, Nyfeler notes that the Gübelin Lab has modified its approach to origin identifications. “We are not changing what we do [i.e., determining country of origin by determining the specific mine], but only how we do it,” writes Nyfeler. “We are now confident that we can manage today’s and tomorrow’s complexity created by the many more mines, and by the overlapping gemological features of gems from some of these mines. That’s all. The result stays the same: country of origin.”

Nyfeler adds, “At Gübelin, we invest a lot [of time and money] in traveling to the mines, in studying the material from new sources, and in evaluating the data in smarter and more reliable and reproducible ways.”

Hänni cites new instruments, experienced gemologists, and a large collection of origin references as keys to managing the challenges posed by the proliferation of mines.

But many gem identification labs are turning to source type as a new tool. It’s based on the fact that most gem corundums can be identified as coming from a particular geological type of gem deposit. That narrows down the potential localities to countries in which those types of deposits are found.

Shane McClure, GIA’s director of West Coast gem identification, discussed a useful method of classification during a presentation at the Gemological Research Conference in San Diego before the recent GIA Symposium. It uses a two-tiered source type: First, corundums can be classified as coming from a particular geological formation; second, they can be classified into groups that have specific properties and internal characteristics. For example, blue sapphires from Madagascar and Sri Lanka may have the same geological formation and internal characteristics.

McClure says that while many different growth environments are possible, those for corundum can be categorized into two main groups: metamorphic and magma related. These two categories can be recognized visually by a knowledgeable observer. He also says it may be possible to further classify rubies and sapphires based on their dominant inclusion features and other physical characteristics. At his GRC presentation, McClure spoke about sapphires that appear as Kashmir both in color and under the microscope, but that can be from either Kashmir or Madagascar.

It has been proposed that the GIA source-type classification laboratory report also include a list of possible country origins where particular types of corundums can be found.

McClure says the method doesn’t try to pinpoint geographic locality or specific deposits. “But it does provide information that directly relates to a stone’s appearance and position in the marketplace,” he writes.

Nyfeler says the Gübelin Lab welcomes the idea of GIA developing a classification system for corundum, but he says there’s a problem: “The name is wrong! Their system has nothing to do with ‘source type.’ What GIA is referring to is not a geologic classification; they address the topic from another direction, which [should] not make any reference to geology or source. For example, the GIA system is having Mong Hsu rubies and Kashmir sapphires in the same metamorphic category. This is fine for a general classification system, but this is [incorrect] when it is called source type, because the lithology [the macroscopic nature of the mineral content, grain size, texture, and color of rocks] and paragenesis [order in which minerals together in rock were formed] of the two host rocks are by no means the same.”