Ram's Head Pods bring a nice natural twist to any bioactive build. The curved shape adds some depth to the forest floor and gives isopods and other inverts somewhere to hide.
 
They're dense and porous, so springtails especially love grazing on them. Since these are naturally collected, expect some variation in size, shape, and colour, and don't be surprised if one arrives in a couple of pieces. It doesn't affect how useful they are in the enclosure.