I was using it whilst mining and I had read about how the chances of finding a gem drop with each gem you find. So as an experiment, *after* I had already collected 6 gems I used Rubeweed and... kept on collecting gems! So it does really work, and now I swear by it.
I just wanted to update everyone about this since I was asking about rubeweed recently and nobody seemed to think it did anything. Well it does you just have to use it under the right conditions I guess. ^_^