A discussion of how inclusion patterns mimic crystallographic symmetry in ruby and sapphire.
Gem Inclusions – How inclusions mirror external crystal symmetry
One of the joys of gemology is the examination of fingerprint inclusions. Particularly when viewed in reflected light, they can shine the light fantastic.
Just how did these fascinating inclusions form? At any point after a crystal grows, it may fracture. Given the proper conditions, that fracture may later heal closed, leaving a scar-like inclusion typically known as a “fingerprint.” Rubies and sapphires often contain gorgeous examples.
The healing process involves exposure to a combination of heat and solvents. In the ground, elevated temperatures and solvents produce healing of fractures via corundum-containing solutions. Dissolved nutrients (solute) may come from solvents dissolving surrounding crystals, the exterior of the crystal itself, or the interior walls of the fracture. This dissolved nutrient material then regrows on the walls of the crack, “healing” it closed. But an internal scar remains, something we term a “fingerprint” inclusion (see Figure 1).
The brightly-colored areas are actually trapped fluids. The cavities they reside in are tiny negative crystals, with miniature crystal faces that face inward, rather than out. Since they are crystal faces, they will have a symmetrical relationship to one another.
From the above, one can clearly see that fingerprints are far from random, but have patterns that relate intimately to the underlying atomic structure. Far from being flaws, they are just what the word suggests – fingerprints – features that are unique to that gemstone alone. No two are ever alike.
About the author
Richard W. Hughes is one of the world’s foremost experts on ruby and sapphire. The author of several books and over 170 articles, his writings and photographs have appeared in a diverse range of publications, and he has received numerous industry awards. Co-winner of the 2004 Edward J. Gübelin Most Valuable Article Award from Gems & Gemology magazine, the following year he was awarded a Richard T. Liddicoat Journalism Award from the American Gem Society. In 2010, he received the Antonio C. Bonanno Award for Excellence in Gemology from the Accredited Gemologists Association. The Association Française de Gemmologie (AFG) in 2013 named Richard as one of the fifty most important figures that have shaped the history of gems since antiquity. In 2016, Richard was awarded a visiting professorship at Shanghai's Tongji University. 2017 saw the publication of Richard and his wife and daughter's Ruby & Sapphire • A Gemologist's Guide, arguably the most complete book ever published on a single gem species and the culmination of nearly four decades of work in gemology. In 2018, Richard was named Photographer of the Year by the Gem-A, recognizing his photo of a jade-trading market in China, while in 2020, he was elected to the board of directors of the Accredited Gemologists Association and was appointed to the editorial review board of Gems & Gemology and The Australian Gemmologist magazine. Richard's latest book, Jade • A Gemologist's Guide, was published in 2022.
First published in October 2005, while I was at the AGTA GTC.