I've done Mother Oak with an income of 2 nature, and the Eternal Pyre with 6 or less income. Just a matter of saving them up towards that goal, vice using them up via forging or summoning. Note that it's also possible to trade gems with other human players - I got a particular kick two weekends ago in a blitz when I used nature gems
from the person who had Mother Oak up to cast my own.
( 4 players - I also traded for about 35 more nature from the other two players. )
And I disagree about them not being worthwhile just because you have a high income already. Okay - an extra 10 nature isn't much if you already have 30+, but on a smallish map they're awesome. And you can always put enough gems in to make it unlikely someone else will be able to dispel or override yours - which means that they've wasted their own gems.
Mother Oak means being able to summon 2 extra castings of lamias a turn, or an extra carrion lord every 3 turns as CW Pangaea. True, it might not stay up long - but if Pangaea and Man aren't in the game, it has a fair chance of staying up.
Eternal Pyre : 20 extra fire gems a turn means ... 300 extra gold a turn, more if you have an alchemist or the alchemist's stone. Or more items, more summonings, etc.
And again - if you're not playing on a huge map, you have a good chance of it staying up a while, and of being able to gauge how likely it is to stay up.