Vital Spark OCT - Spark Gems

QUESTIOOOON, just how breakable are the gems? I know they'll have to be pretty study so they don't accidentally smash in someone's pocket, but about how much force are we talking about to break them? Like if I put a Spark Gem on a large boulder, and hit it with a smaller rock, would that break it? XD Or could it be broken by dropping it from a great height onto a hard surface, in which case about how high? Also if a Spark Gem falls into some kind of... hideous and destructive concoction like lava or something, which would no doubt damage it at least, would that also count as breaking it and the life force will escape?