Aethril

What I have gathered from my person experience:
-The proc rate seems to be quite low ~5%, maybe a bit higher.
-I have only ever seen it proc off herb nodes (i.e. herbing dead plant enemies doesn't cause it to trigger)
-The spawned herb will always be identical to the herb you just gathered (i.e. proccing on dreamleaf will always spawn another dreamleaf)
-The spawned herb will spawn immediately and make a distinct sound when it happens.
-The spawned herb for all intents and purposes acts identically to a normal herb node, gives the same amount of herbs and can even proc additional herb spawns if you are very lucky (I have gotten a chain of four in a row).

i farm almost 10k [Aethril] for Rank 3 and still nothing :D Dont give up guys

I think that's a pretty valid reason to give up ;)