[Sapphire]

Seconding Nature. I would definitely at least throw a few points in earth enchantment and volatile orb before doign Nature. But you can grab Heart of Oak at a relatively low level. Just don't put more points in Nature than that until you've maxed Earth and pumped up your core dweller some and gotten eruption.

Remember, in Normal and early Epic your damage skills are where to put most of your points. Defensive and utility skills are most helpful mid-epic through Legendary.