I think medium bark or a mix of medium bark and sphag, which is what I like, is a good medium for someone just learning to grow these. Rocks or almost anything are just fine but it takes a bit of experience to understand how to care for them first. The bark/sphag adds moisture but should keep it open and airy too in a good pot to allow for a person to get the hang of it without killing the orchid in the process. Just my