This content was first published on IGR – Italian Gemological Review no. 21 in 2025. The information provided here is therefore current as of the original publication date.
So-called “geuda” sapphires are varieties of corundum naturally rich in fine rutile filamentous micro-inclusions, which give the rough material a milky or hazy appearance, making them, in their original state, unattractive for the gemstone market. Abundantly found in the deposits of Sri Lanka, these stones were traditionally used as garden filler or gravel due to their low commercial value.
The scenario changed dramatically when Thai traders, initially purchasing geuda sapphires at low prices for decorative purposes, discovered their aesthetic potential after exposure to high-temperature heat treatment. This marked the beginning of a true commercial and technical revolution. The classic treatment technique involves heating the stones to temperatures between 1600 °C and 1800 °C, often in a reducing atmosphere, to dissolve the rutile inclusions and improve transparency and blue color saturation.
For over forty years now, vivid blue sapphires obtained from geuda rough have occupied a significant portion of the international gemstone market. In this context, it has become increasingly important for gemological laboratories to identify and trace back these materials to their geuda origin.
A recent study published in Solid Earth by Pluthametwisute, Nasdala and colleagues developed an innovative approach for reliably distinguishing natural “geuda” sapphires from heat-treated ones, using a combination of luminescence testing, microscopic observation, and spectroscopy.
The study revealed that natural sapphires emit orange-red luminescence under long-wave UV light (365 nm), while heat-treated stones exhibit blue luminescence under short-wave UV (225 nm), providing a clear visual indicator of thermal treatment. Another diagnostic clue is the transformation of the original needle-like inclusions typical of the “silk effect”: exposure to high heat causes these needles to fuse, forming disordered “blue spots” under the microscope, which serve as clear evidence of heating up to 1650 °C.
Furthermore, the research highlighted the limitations of classical techniques: for example, FTIR spectroscopy alone proved to be not entirely reliable for definitively distinguishing treated from untreated sapphires. As a result, the researchers propose a combined diagnostic strategy that includes: observation of luminescence under different UV wavelengths, microscopic analysis of the inclusions and their transformations, spectroscopy to detect the intensification of the absorption band at 580 nm.
This multidisciplinary approach represents a significant step forward in the ability to “trace the material’s origin” and more accurately evaluate the thermal history and gemological provenance of seemingly identical sapphires. For more details, you can read the full article at the following link: https://se.copernicus.org/articles/16/81/2025/
For further reading:
https://www.ruby-sapphire.com/index.php/about/10-articles/830-brief-history-heat
https://www.gemologyproject.com/wiki/index.php?title=Heat_Treatment
Gem news published on IGR – Italian Gemological Review #21 – Autumn 2025.
