def. Ulexite:
1. Hydrated sodium calcium borate hydroxide (formula: NaCaB5O6(OH)6•5(H2O) ), a silky, brittle, generally white evaporate mineral  which often crystallizes in the form of densely-packed fibers that transmit light along the long axis of the mineral.

2. A party trick rock. Have any party guests who think that geology isn’t awesome? Just pull out your fibrous ulexite sample and say, “Hey look, I have a fiber optic rock.” Then watch the fun– the geology fun– begin.

 

Ulexite really does have remarkable optical properties, as the photos below demonstrate. Personally, I think that the fiber optic images produced by ulexite are even more fun and interesting than the double images caused by refraction in calcite.

I bought my ulexite sample at a rock sale. Does anyone know of good places to collect ulexite in the field?