Royal Dreadstone is good if you have problem with mana regen in combat . Purified Dreadstone is still a bit superior since you will get a bit more spell power (with tree of life of course)and mana regen .
Gemming for druids depends on your role as a healer, gear and your spec, At the moment, rejuv +wildgrowth seem to be the bread and butter. In order to really punch these spells up, hitting the soft haste cap while not impacting spell power is the balancing act.
Druids have inherent big mp5 regen and our gear bonus' tend to give us the rest so you never want to gem mp5.
I would suggest the following;
Meta (head) : Insightful Earthsiege Diamond. This procs quite a bit and is very useful if you are having any mana issues on longer boss fights.