O_O How do you earn gems??

I'm assuming you're talking about Diamonds? If so... you can't really earn them, per se. They are the currency bought with real life money. You can buy them from the Diamond shop, or you can purchase them for gold from other players on the Currency Exchange (under the trade tab). Diamonds are also, rarely, given out through games like the advent calendar or things like the Haunted house during festivals.

If, in fact, you did mean Gems, these are a currency from another game entirely (usually Flight Rising) that people trade for items and currency on Sylestia.

Whups, yeah, I meant Diamonds, FR has screwed me up XD
Hmm, okay, thanks~