"Dave" wrote in message
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#232 The mineral ulexite

Correct, it's also called a TV stone, if you hold it up to your eye you
can't see through it (you can't see across the room, just a blur of light),
an image can be seen through it only when it's placed directly on something.
With it's fibrous crystal growth, it's like naturally occurring fiber
optics, as you can see in the photos the image is transmitted straight up
from the bottom. If you put it on a dollar or a printed page and move it
around, the way the image looks in relation to the real printing is very
unusual. I get really great reactions with this one, it's one of my
favorite things to show people.