Fun with ulexite

I recently acquired a nice specimen of ulexite:

"Ulexite is also known as TV rock due to its unusual optical characteristics. The fibers of ulexite act as optical fibers, transmitting light along their lengths by internal reflection. When a piece of ulexite is cut with flat polished faces perpendicular to the orientation of the fibers, a good-quality specimen will display an image of whatever surface is adjacent to its other side"

In other words the image appears to be on the upper surface, not under the rock the way it would under a piece of glass:

In this enhanced close-up you can see the ends of the long, optic fiber-like crystals:

By chance I discovered that when a point source of light is transmitted through the specimen a distinctive pattern of two concentric circles appears. Here I've used a well collimated laser to demonstrate the effect (the granulation in the image is caused by interference between photons of coherent light and is typical of reflected laser light in general):

The brightness and diameters of the rings varies depending on the section of the specimen transmitting the beam. Tilting the specimen causes the rings to stretch out coma-fashion. Regardless of the precise orientation there is always a central dot or ring and a single, dimmer outer ring.

Update: I found the explanation for this behavior here.

I obtained my specimen on eBay from whereonearth. I can highly recommend them. They have some really bizarre stuff for sale at their web site.

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