Treebeard

I've also seen these rocks around in various pet shops. Some of them have been dyed or painted bright colours and wouldn't suit but the unpainted ones would look great with root over rock or a small group planting on them.

Another idea, along the same lines, is big lumps of driftwood, often sold at aquatic retailers. My local one has quite large pieces (12" across) in wierd and wonderful Kimura-esque shapes.
Obviously not suitable for root over rock, but very suitable for phoenix grafts.

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