Microwave brings rubies to shine

Unconventional method improves gloss and color of gemstones

Ruby from Tanzania - but these crystals are not always that colorful and bright. © Rob Lavinsky, iRocks.com/ CC-by-sa 3.0
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Beaded Gems: Researchers have discovered an unusual recipe to upgrade rubies of inferior quality: just put the gems in the microwave for a short time and they'll shine in new splendor. Analyzes show that rapid heating optimizes the crystal structure of the rubies and improves their light absorption. As a result, dirty-brownish specimens turn into bright red shiny.

Rubies are coveted precious stones because of their reddish shimmering color, but are also used for lasers and sensors. Purely geologically, these crystals are nothing but "polluted" corundum: an aluminum oxide (Al2O3), in whose crystal lattice chromium atoms are embedded. As geologists found out a few years ago, rubies only form where two continental plates of earth collide, for example in the Himalayas. Under high pressure, the red corundum develops there in ten to 40 kilometers of depth.

Dull-brown instead of bright red

Rarely, however, are rubies as pure in color and bright red as the jewelry manufacturers would like. Often they have bluish spots or are only very dark red. But there is a remedy: Treating the gems with lasers, certain chemicals or high heat, then changing the electron arrangement in their grid and thus the light absorption.

An entirely new method to bring rubies to new radiance has now been discovered by Subhashree Swain of the Indian Institute of Minerals and Materials Research and her colleagues: Microwaves. For their study, they used several rubies, which were found in Sinapali in northeastern India. The between five and eleven millimeters small gemstone chunk had a reddish-black color and were therefore considered inferior quality.

Miraculous Transformation: Sinapali Ruby Before and After Microwave Treatment © Subhashree Swain / Applied Physics A

Transformation in the microwave

The researchers put the rubies in a microwave with three megawatts of power and heated them in about 50 minutes to a temperature of 1, 500 degrees Celsius. They then analyzed the structure and light absorption of the stones using X-ray diffraction, UV spectroscopy and Raman spectroscopy, among others. display

The result: Even at first glance it was obvious that the rubies appeared brighter and more homely after the microwave treatment. This was also reflected in the measurements: "The absorption at 560 nanometers has increased significantly, as well as the total light absorption in higher wavelengths of 600 to 800 nanometers, " report Swain and his colleagues. Because this filters out more dark red and near-infrared, the stones appear brighter than before.

Changes in the crystal lattice

These changes go back to the smallest changes in the crystal lattice of the rubies, as the researchers explain. According to the measurements, the irradiation changed the electron distribution of foreign atoms such as chromium, iron and titanium. In addition, defects and irregularities in the crystal lattice were remedied by rearrangements. This increased the transparency and color purity of the rubies.

"Microwave heating is an unconventional method for rapid thermal stimulation - but it does produce good results in gems like rubies, " says Swain. This improves the previous possibilities to optimize rubies by subsequent treatment. Because the microwave is faster and smoother than other forms of heating. (Applied Physics A, 2016; doi: 10.1007 / s00339-016-9703-9)

(Springer Science and Business, 03.03.2016 - NPO)