When you made the Crystals for combining two of the same Shards you just add a double effect in the card.
I think it should be a little less. Like, two Crystal of Gratitude healing for 7 or 8 HP.
Then why would you bother combining them? If a single crystal is weaker than two shards, then you should just take the two shards and double their effects, instead of taking the crystal.
Well, I guess one use is to take BOTH the shard and the crystal. Say, you can only have six Shards of Gratitude in your deck, but along with the six shards you can have some Crystals of Gratitude.
But still, I don't think they're that overpowered. If their effects are weakened, then their numbers shouldn't be limited to less than six.
Should the Crystals and Gems be limited? It'd be pretty OP if you have six Gems of Gratitude that each heals you for 20 HP per turn. If they're not limited, then the effects of the primary colored gems and crystals should be weaker than their individual shards combined.