If you already have an Ivy King, you can get 100 Vine Ogres for the price of one Tarrasque.
In addition to Ivy Kings and Vine Ogres, here are some things I like to get with Nature gems:
- Haunted Forest
- Lamia Queen (and Lamias)
- Faery Queen
- Firbolg
- Couatl
- Wild Hunt
- Gift of Health
- Gift of Reason
- Ring of Regeneration
- Endless Bag of Wine
- Thistle Mace
- Soulstone of the Wolves
- The Chalice
- The Tome of Gaia
- Boots of Calius
- Boots of the Messenger
- Robe of Calius
- Hydra Skin Armor
- Totem Shield
- Eye Shield
- Vine Shield
- Snake Bladder Stick
The Tarrasque isn't a bad unit. I just have a long list of what I consider better uses for Nature gems.