Cultural Treasures, UK edition: The Natural History Museum


I had occasion to visit the museum of Natural History yesterday, and it is really really really good. Not only does it have one of the best collections of fossils in the world, it also has a truly incredible mineral collection. You can see valuable Gems in their natural and cut states. Anyway, among these various exhibits I have discovered a very special treat. Here is a picture of a Gem known as Alexandrite:

These are actually the same Gem in slightly different ambient lights.

One Gem or Two?

Alexandrite is fascinating because it changes colour based on ambient light conditions. There are two effects at work here. The first is called Pleochroism. This occurs when a crystal has an anisotropic structure, which means that it responds differently to photons depending on the orientations of the Electric field. This is known as the polarisation axis of the photon. Daylight is partially polarised, and hence, anisotropic crystals look different colours when viewed from different angles. Here is an example of a strongly Pleochroism topaz, from the Wikipedia page,

Changing the camera angle results in a different colour, since it changes the relative angles of the polarisation axis of the photons and the optical axes of the crystal.

However, Pleochroism, while interesting in its own right, is not Alexandrite’s most distinctive feature. It also changes colour under unpolarised  light in the right conditions. The picture taken above has the same gem under Daylight (green) and an ordinary tungsten filament lamp (what you have in your house….probably). The reason for this is that Alexandrite absorbs extremely strongly in the yellow region of the visible spectrum, about 600nm, with a very narrow band. If we compare daylight to a tungsten lamp, we obtain

Lots of different lights.

and by looking at the transmission spectrum of Alexandrite we get

See that peak at 400-500nm, exactly where tungsten and daylight have their maximum difference.

so we see that since sunlight has significantly more blue (400-500nm) in it than tungsten, it strongly affects the colour balance of the stone, and blue and yellow together make green. Under Tungsten lamps, the blue almost disappears, and we end up just on the red side of yellow.


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I know I live in my own world, but I like it: they know me here.

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