Hambergite

Very fragile and brittle, so only faceted for collectors, when cut it is glass-like, but with high birefringence, causing a double image.

Named after Axel Hamberg, a Swedish mineralogist, who discovered it in Norway, although it has since been found in other locations around the world.

Harder than quartz it would be ideal for jewellery, but it doesn't cleave/cut cleanly, leaving small flaws in faceted stones.