Sponge rock #3 (perlite)
Sponge rock, also known as Perlite, is a great ingredient for do it yourself orchid potting mixes. It's light weight and inert, so never breaks down (although it does crush). And while it is non-moisture retentive, it's full of all sorts of little holes and crannies, which hold water that is then released slowly as it evaporates. And due to the odd and varied shape of each piece, they don't compact together, so allow for plenty of air flow.
It is recommended that when working with sponge rock, you wear protective clothing, a respirator and work outside in a well ventilated area.
#3 size is appropriate for seedling and medium size orchids mix. One gallon.