These were made with the slightly more expensive dental plaster, I have to really recomend this over regular plaster. The blocks are very hard but not real brittle, in fact I dropped several from my table to a wooden floor and didn't break any of them, Never would have happened with regular plaster, at the very least they would have gotten chipped up or possibly shattered all over the place. Making them can be a time consuming experience, dont' waste your time with reg plaster.