While their I couldn't differentiate between the pieces of rock, but I understood from Henk Bos's documentation that they can be heat treated, so of course I couldn't resist to throuw one in a camp fire (I picked up 4 pieces, to be certain on one hand and not to manny so my girlfriend wouldn't start to worry). The piece of rock on the campfire did get lighter and a lot softer but broke apart due to I guess the thermal stress entering an already hot fire