Is there a way to get more gems or just by buying them?

You can randomly earn gems from completing skirmishes( can be 1 of the 3 rewards amount seems random but I've won 70 at once though), they rarely drop from chests (but only 5 at a time), also reward for completing most quests (book button above map)

Probeer zo snel mogelijk de battle`s te winnen, hoe sneller hoe meer Gems. Upgrade je wapens en probeer gems te sparen. Bij ieder nieuw level krijg je schatkisten, verkoop die niet maar zet ze ergens op `n eiland want ook deze kun je 2 dezelfde bij elkaar doen tot wel de grootste en die kun je dan kopen voor veel minder dan in de winkel. Blijf wapens upgraden of desnoods verkopen voor bronzen wieltjes om een ander wapen te upgraden. Probeer je gems te sparen totdat je genoeg hebt om de grootste schatkist te kopen die je dus hebt neergezet op een eiland.

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Try to win the battle as fast as possible, the faster the more Gems. Upgrade your weapons and try to save gems. With each new level you get treasure chests, do not sell them but put them somewhere on an island because you can also put 2 of the same together up to the largest and you can buy them for much less than in the store. Keep upgrading weapons or if necessary sell them for bronze wheels to upgrade another weapon. Try to save your gems until you have enough to buy the largest treasure chest you have placed on an island.