....optical property called birefringence.

"Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled."

At least that's what it said, funny I've always called it by the other name, coz it was the only one I knew could remember accurately.

Dropped into the Clickery whilst on the weekly meat-hunt. While She Who must be Obeyed catches up on the crack I tend to prowl and this crystal is one of my regular play-stations. They fascinate me.