You would need to assemble a Kairos dragon through the perch of Kairos. This is, to date, the only dragon not gained by breeding. You have to collect artifacts through three different stages.
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There are no legitimate sites or apps that give you DragonVale gems, besides the actual official DragonVale game app.
if you enter the code: 10084572 you get 45 free gems
You can't exchange coins for gems. You can only exchange gems for coins. I don't know why, but the DragonVale company won't allow that to happen.
You get 5 gems for winning
if you enter the code: 10084572 you get 45 free gems