Herb Seed + nothing. No crystal needed. I planted some Herb Seeds but ended up missing the feeding period. I ended up getting 5 (or maybe 6) Amaryllis from one of the 5 plants.
The first time I tried this (herb seeds and a dark crystal in an earthen flowerpot), I ended up with a deathball (I don't remember when I fed the plant the crystal).
My second attempt with the same recipe to grow a deathball netted me 5 amaryllises (amaryllisi? LOL). I fed the plant the crystal on Lightsday.
This is going to sound silly, but I planted 2 pots of herb seeds at once, I fed one a Light Crystal, and One a Dark Crystal. One plant (I didn't check which >_<) yeilded 10 Mahura Garlic, the second, 5 Amaryllis. Food for thought.
If your in juneo you can just buy it, if not buy it at the auction house, people usually overprice it like 200 or so after buying it for 100 because the know about this quest