This may be a bit farther out than you were seeking, but I direct you to [Pyura Chilensis][1]

This is not a plant, but it is feasible that a more plant-like organism could similarly form a hard rock-like shell from mineral debris in its environment.

This is a filter feeder that looks like a chunk of rock with internal organs; I cannot easily find information on the hardness / composition of that shell, but it looks to be potentially similar to that of oysters or barnacles. 

  [1]: https://en.wikipedia.org/wiki/Pyura_chilensis