Unusual photochromic faceted glass
Gemology is a discipline that often bring some amazing and unusual materials on the path of the curious gemologists. A while ago I had the chance to study a faceted man-made glass (6.60 ct – see picture below) that shows a specific effect sometimes encountered in some rare natural minerals: a reversible photochromic effect [1]Milisenda C. C., Koch S., Müller S., Stephan T. and Wild M. (2015) Gemstones with photochromism, Proceedings, 34th International Gemmological Conference IGC, Vilnius, pp. 107–109 (a.k.a tenebrescent effet).
Photochromism is a property of materials that change color – or darken – upon exposure to light. A mineral that possesses this property is said to be photochromic. A reversible photochromism means that the material gets back to its original color once the light is removed (usually between few minutes to few days, function of the nature of the material).
With a R.I. of 1.52 and a specific gravity of 2.38, this inclusion-free faceted glass has the ability to change color as well as to darken automatically when exposed to sunlight.
The picture below shows with arrows the different steps of the change of color:
- When just pulled out of the dark, the facetted glass appears light greenish yellow,
- After 2 minutes under direct sunlight it appears light brown,
- After 5 minutes under direct sunlight it appears dark brown,
- After 10 minutes under direct sunlight it appears grey, and will stay grey as log as it stays under the sunlight.
As it is a fully reversible process, after few minutes of being kept in the complete darkness its color will go back to light greenish yellow.
We all know this synthetic material as being used on “light-adaptive” sunglasses [2]Erickson B. E. (2009) What’s that stuff? Self-Darkening Eyeglasses – The science behind dual-purpose lenses, Chemical & Engineering News (American Chemical Society), Vol. 87, No. 15, p. 54 … but someone has the interesting idea of faceting it ! For which purpose … who knows, but at least it has been fun studying – and sharing here with all of you – such a gemological curiosity.
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