This content was first published on IGR – Italian Gemological Review no. 19 in 2024. The information provided here is therefore current as of the original publication date.
In a recently published study on Crystals, Volume 14, researchers examined two rare andradite garnets that exhibit an extraordinary ability to change colour. These minerals can change from a yellow-green hue in daylight to a purple-brown under incandescent light, and from a yellow-brown to a red-brown.
Using conventional gemological instruments and advanced spectroscopy, including infrared, UV-Vis-NIR and Raman spectroscopy, the researchers explored the gemological properties and colouring mechanisms of these garnets. The results of the experiments revealed that the studied specimens are andradites, with a very low presence of chromium and a high concentration of ferrous iron, the main responsible for the colour change.
The most distinctive feature of these garnets is the presence of two ‘transmission windows’ in the visible region of the UV-Vis spectrum, specifically in the green (at 525 nm) and red (above 650 nm), indicating that the material does not absorb light at these specific wavelengths, which can influence the perceived colour of the material. This phenomenon, known as the alexandrite effect, is the main cause of the colour change of these garnets.
Furthermore, spectral analysis revealed that the minerals in question show similar behaviour to andradites from Namibia, indicating a possible origin of the material.
The full paper is available on: www.mdpi.com/2073-4352/14/2/180
Gem news published on IGR – Italian Gemological Review #19 – Autumn 2024.
