I've never attempted to break them or anything, but they are not incredibly brittle. I would say they feel similar to a rough quartz crystal. I'm sure they would break easier than quartz if hit with a hammer or something though. The large pile of crystals in the pictures are stored like that in a paper bag, and nothing has happened to them.

The one thing I have been warned about with these though is humidity. Apparently high humidity can make them cloudy. I guess if you had some really nice ones, they could be stored with some silica gel.