Eduard J. Guebelin, whose pioneering study showed that “flaws” disclose the origins of diamonds and other precious stones, died Tuesday in Lucerne, Switzerland. He was 91.

Guebelin’s laboratory originally was intended to inspect and guarantee the gemstones in jewelry sold by the family firm, but famous auction houses soon started requesting that it report on important gemstones coming up for sale, a company statement said.

Gems & Gemology, the quarterly journal of the Gemological Institute of America, said in a 2003 tribute that “inclusions in gems were generally considered disagreeable ‘flaws’ or ‘imperfections”‘ when Guebelin started his research as a young man.

“Today, however, gemologists almost instinctively recognize the diagnostic importance and natural beauty of inclusions,” the magazine said. “This evolution in itself is a tribute to Dr. Guebelin’s remarkable career.”

His numerous publications included a 1953 volume that provided “the first systematic classification of inclusions in diamonds, colored stones and synthetic gem materials,” Gems & Gemology said.

“With more than 250 photographs, it demonstrated how inclusions could help reveal the identity and source of gems,” it said, adding that the book “became a classic, changing the face of modern gemology.”