Multispectral gemstones

[Hornblende]

Hi all,

Here is the result of a multispectral experiment involving a set of red and blue gemstones. I wanted to know how gemstones of similar color appear in ultraviolet and infrared, and if that could be usefull for identification, finding fakes, treatments etc.

 

Gear used:

Camera: Canon 6D full spectrum

Lens: El-Nikkor 80mm f5.6 and Carl Zeiss Jena 50mm f2.8

Filters: Baader-U ; Lifepixel IR 830nm filter ; Lifepixel UV/IR cut filter ; Lifepixel Super-Blue filter (BG-3)

White PTFE was used for white balance

Sunlight for lighting

 

 

Red gemstones

 

VIS – The rubies and spinels are colored by trace amount of chromium, the garnet is colored by Iron, and the various glass by either trace amount of selenium, erbium, praseodymium or niobium.

 

IR – All stones appear relatively transparent to IR except the selenium glass

 

UV – the spinels, garnet and selenium glass are UV-dark. The Niobium and Praseodynium glass seems to absorb the shortest UV wavelenght, and the Erbium glass seems pretty transparent. The rubies are light to dark with a greenish color, does that mean they absorb the longest UV wavelenght??

 

 

Blue gemstones (Without UV image, it is too cold outside :( )

 

VIS – The aquamarines are colored by trace amount of iron, the sapphires are colored by trace amount of iron and titanium, the turquoises and azurite are colored by copper, the tanzanite is colored by trace amount of vanadium, the small spinels are colored by trace amount of cobalt and the lapis lazuli is colored by trisulfur radical anions.

 

IR – The aquamarines and turquoise are strongly IR-absorbent! On the other hand, the lapis lazuli, tanzanite and spinel are IR-transparent. The sapphires and azurite don't seems to change much compared to the VIS image.

 

Blue+IR – Using a BG-3 filter greatly emphasize the IR-absorbance of the aquamarines and turquoises, which show a very nice vibrant dark blue color. The sapphires and azurite, again, don't change much compared to the VIS image.

[Cadmium]

Very nice and interesting indeed! :-)

[Andy Perrin]

Cool! The selenium glass may be absorbing more in IR, but to me it also looks quite transparent - one possibility is that it has a greater refractive index in IR than the others and what you are actually seeing is the bent image of the dark surface underneath. Easy way to check is to put them on an IR-white surface and try again.

[SteveCampbell]

Well done! The difference in the aquamarine is striking

[Shane]

I am always interested to see this kind of work. However, as I mentioned before when you were presenting IR luminescence and gemstones, the same applies to reflected UV and IR (as well as transmitted UV and IR)....it requires a large amount of reference gemstones from different locations. If you end up doing the DUG with Emmanuel then this will be less of an issue.

 

With regards to UV transparency and gem identification, you might be interested in this article.

https://www.gia.edu/gems-gemology/spring-1999-separation-natural-synthetic-colorless-sapphire-elen by myself and E. Fritsch.

 

You can also download the entire Gems & Gemology issues from the GIA website. It will provide a significant introduction to gem identification also give you an insight into some of the issues with regards to obtaining sufficient quantity and sourcing from known verifiable locations. Of course, while humans like to pigeon hole everything into neat little categories, nature prefers to mess with us by throwing curve balls that don't always appear to fit the data.

[Hornblende]

Thanks all :)

 

Shane,

I read your paper, it is very interesting! I understand that I need a lot of reference stones and a calibrated source of light, I don't have any of those for the moment, (haaaaaaaaa :(), so I am just messing around with some stones from my collection. I already saw interesting behaviours such as peridot being completely black in IR!

I am still thinking about doing a DUG but for the moment I have to finish my Master's study in Geology and try to get a job. I'll keep you updated about my future work on gemstones!