The only Type Gem legally obtainable past Generation V is the Normal Gem. In X & Y, it's infinitely renewable by checking in Terminus Cave from time to time. Several event Diancie also came holding a Normal Gem. (Source)
The other seventeen Gems exist in the game's code, but they're unable to be found without hacking them in. You're unable to transfer items through PokeBank, so it's impossible to transfer the Gems up from Generation V. You're out of luck if you want anything other than a Normal Gem in X & Y. (Source)