I used to use a chip from my stack to protect my cards until I visited Tucson's Gem & Mineral show this year. I found a piece of spectrolite (Finnish labradorite) that's very unusual, very pretty, and is just the right size.
This isn't my piece, but it gives you the idea. Mine actually has more red and green in it.
For physics/optics geeks: Can you explain why you can only see the colors when you hold the stone at one particular orientation (assuming one light source)? If you rotate the stone 180 degrees, you can't see the colors and it just looks black so there's something going on besides a linear grating effect. Same for all other orientations except one. Some kind of retardant/thin film phenominon?