vermiculite wich is heated volcanic rock. I would use half and half. Perllite wich is heated mica is also a good one. It looks like cork lol. The problem is there very light. You can add some rock salt or sun gems. I have found there all good at holding the scent. What i have found is they dont last for ever. 2 Months is good. In my opinion. Another use for vermiculite is vacuum cleaners also.