So, I've noticed these little adds for cards you can buy that double the money you get in battle while you have it in your collection.

How exactly does this work? From my understanding the card cannot leave your collection, so that's cool. Can you buy only one of each gem or multiple? and if you can buy more than one how does the effect stack? It could double the base amount twice (giving you 3X for two) or double the totaled amount (4X crystals). If the earlier is the case how does it stack with my bonus crystals from my clan?

The card has no time limit.
You actually can't sell this card (remember that the posibility t sell unsellable cards is coming).
You can buy only one exemplar of each gem.
At the end of each game you played, you receive twice the amount of crystals and twice the amount of XP if you have the two gems.
This stack with your clan bonus for XP or crystals.

When I do a perfect with full bonus, I earn 138XP and 184cx.

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Collectionneur de cartes

I wish people looked for solutions and answers before re-posting the same questions :p

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Eredan Kickstarter

Does it work in adventure mode?

Yes, all crystals received by playing the game is doubled, against AI or against others players.

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Collectionneur de cartes