Hi I was wondering if anyone knew what stage in the game that we can get..

You can get the last job classes after you have collected the four fangs (wind, water, fire, earth). It is highly recommended that you do not get them yet though, because you probably need to level up a little more before you can face the boss that guards the crystal. (Around Lvl. 40 should do)
To get to the Crystal you'll need the Invincible. Go to the northern part of the continent with Amur on it, and you should see a narrow passage between mountains with statues on either side. You have to get off the ship and walk through the passage to make the statues disappear, get back onto the Invincible and continue to fly. (The way should be obvious) Then enter the large, square building and go straight ahead into the door. The Crystal should be in that room.