The Ruby is an item that is needed within
Pokemon FireRed in order to fix
Celio's Machine on
One Island during the
Post Game Story. This machine will allow players to ultimately
Trade with those playing
Pokemon Emerald via the
Wireless Adapter, but it will require a player to find both the Ruby and
Sapphire before being able to do so.
Out of the two crystals, the Ruby is much easier to obtain than the
Sapphire. The Ruby is actually located within
Mt. Ember on the northern portion of
One Island, but, again, it cannot be found until you have defeat the
Main Story. There's actually an entrance to go within Mt. Ember that will easily be noticeable since two
Team Rocket Grunts will be blocking it. After defeating them, you will be able to access. Once inside Mt. Ember, you will have to solve a few
Strength Boulder Puzzles in order to go deeper and deeper in the cave. Eventually, you will find the Ruby sitting in a room all by itself.
Once the Ruby has successfully been obtained, return it to
Celio to within the
One Island Pokemon Center. From there, you will be tasked with finding the much more elusive
Sapphire.