Nice card but i think it's hard to balance... -1 cost could permit lot more possibility's for a rainbow deck.
I'm not so sure this will do much to rainbow decks themselves, but the relatively high cost will necessitate a combo with nova, QP, or immolation.
Otherwise I think it will see more use in mono decks than rainbows.
The most obvious tactic would be to sprinkle in nova or immolation and use the extra quanta to power casting the gems... thats not really rainbow, its just a deck that needs more power than a pillar provides but isn't to picky about where it comes from.
Consider, for instance, the impact on fire decks.
An immo rush fire deck will be devestating with these ... maybe too much so. Rather than spamming the creatures right off the bat, a handfull of these will be played. with 3 of them, a lava destroyer, fire buckler and firestorm would cost 2
... 2 immos, 2 of these and shards of bravery will make for a devestating acceleration of an already high speed deck... Others can be accelerated with nova instead, but with slightly lower bang for your buck.
The kicker to balancing these is that it severely tweaks cost balancing when multiple are stacked. Balancing, therefore, will need to consider the ramifications to all cards with a current cost above 2.
A good starting point is to look at what cards can be brought down to the minimum. E.g. those with a cost between 3 and 8... Thats an awful lot, so for an easier start, we can narrow this down to cards with costs of 3 and 4 (since on average its hard to get more than 2 to 3 copies of a card consistently)... I'm not even sure where to start going past that though aside from noting that 1 to 3 of these will effectively halve the cost of many (if not the majority) of mid cost cards in the game
At any rate, my point is that this card will have very long reaching balance effects that need to be considered.
