How Infrared Light Reveals the Truth About Gemstones

Collection of rare and exquisite gemstones for gemology studies, Udomner © Sampao-chronicles-stock.adobe.com

A Light Beyond Sight: Infrared in Gemology

Since the early 1800s, infrared (IR) light has been explored for its diverse scientific uses, but in recent decades, it has become especially powerful in the world of gemology. In a newly published study in Gems & Gemology, authors Christopher M. Breeding and Nicole J. Ahline of the Gemological Institute of America (GIA) in Carlsbad, California, outline how infrared spectroscopy, particularly Fourier-transform infrared (FT-IR) spectroscopy, has transformed the way experts identify and analyze gems (1). In general, absorption spectroscopy has become an essential tool in gemstone analysis and authentication (1–3).

How IR Spectroscopy Works

IR spectroscopy works by measuring how atomic bonds within a mineral vibrate in response to IR energy. These vibrations reveal distinctive patterns, or “fingerprints,” that help scientists determine a gemstone’s composition, structural defects, and whether it has been treated or synthetically produced. As Breeding and Ahline note, “No gemologist’s toolbox is complete without an understanding of the power of infrared spectroscopy” (1).

Moving Beyond Visual Inspection

The researchers emphasize that the roots of gemological analysis lie in visible characteristics—refractive index, specific gravity, and fluorescence, among others—but it is spectroscopy that allows gemologists to peek into a stone’s atomic structure. Among spectroscopic methods, infrared stands out for its ability to detect imperfections and impurities that subtly affect a gemstone’s appearance and origin (1–3).

A Technological Leap: The FT-IR Era

Although IR spectroscopy dates back to the 1950s with the introduction of slow, dispersive instruments, the real breakthrough came with FT-IR. Enabled by the 19th-century invention of the Michelson interferometer and a 1966 mathematical breakthrough that allowed rapid data processing, FT-IR became commercially viable by the 1970s and a staple in gem labs by the 1990s. The GIA itself adopted FT-IR in 1986 and has since used it to analyze a broad variety of gem materials (1).

Why Mid-Infrared Matters

The study explains that mid-infrared (mid-IR) spectroscopy, spanning from roughly 2000 to 333 cm⁻¹, is the most widely used infrared spectral region in gemology. The authors describe how FT-IR provides valuable insights without damaging samples and requires minimal preparation. Its capabilities are especially robust when analyzing diamonds—allowing detection of nitrogen-based impurities that define diamond types—as well as identifying features related to heat or chemical treatments in stones like ruby and sapphire (1).

Revealing Hidden Treatments

FT-IR spectroscopy is also instrumental in identifying clarity enhancement treatments. For example, the technique can detect polymers or oils used to fill surface-reaching fractures in emerald, jadeite, and turquoise. This is possible because FT-IR reveals the vibrational signatures of materials inserted into a gem’s microscopic cavities (1).

Defects That Define Beauty

The authors make a point to clarify that terms like “defects” and “impurities,” though often seen as negative, actually play a major role in creating a gemstone’s beauty and uniqueness. Trace elements like chromium in corundum produce the red of rubies, while just a few atoms of boron among billions of carbon atoms result in the deep blue of the Hope Diamond (1–3).

A Gemologist’s Essential Tool

The article is not meant to serve as a comprehensive manual on FT-IR analysis, but instead aims to educate gemologists and gem enthusiasts on the core concepts and modern uses of the technology. Citing previous foundational studies, the authors place their work within a growing legacy of research that confirms FT-IR as a cornerstone of gem identification (1).

As highlighted by the authors, FT-IR spectroscopy has become essential in modern gemological practice, providing rapid, accurate, and non-invasive insights into a gem’s internal world. From confirming the presence of natural features to detecting synthetic enhancements, infrared light continues to illuminate the hidden truths locked within some of the world's most prized minerals.

Reference

(1) Breeding, C. M.; Ahline, N. J. Infrared Spectroscopy and Its Use in Gemology. Gems Gemol. 2024, 60 (4), 474–492. DOI: 10.5741/GEMS.60.4.474

(2) Thaneeshan, R.; Thiruketheeswaran, H.; Aanantharasa, K.; Arulpirathasan, P.; Jaliya, C.; Murugaiya, R. Machine Learning-Based Gemstone Classification through Analysis of Absorption Band Spectrum Images and Refractive Index. In 2024 International Research Conference on Smart Computing and Systems Engineering (SCSE); IEEE, 2024; Vol. 7, pp 1–6. DOI: 10.1109/SCSE61872.2024.10550766

(3) Jin, S.; Renfro, N. D.; Palke, A. C.; Ardon, T.; Homkrajae, A. Application of UV-Vis-NIR Spectroscopy to Gemology. Gems Gemol. 2024, 60 (4), 456. DOI: 10.5741/GEMS.60.4.456