Okay, we all know that the crystals are back in FFIX. But many people say that they don't have many importance in the story, because they only appear in the end.
Because of this i will show to you guys a quote of a guy in another forum, that readed the FFIX Ultimania Guide, and explain some things about the meaning of the crystals, and Garland's plan, in FFIX universe:

"Yeah, in the era prior to Terra needing to devour other planets. Terra only seeked out other planets for assimilation because it started to die. Ever see Fullmetal Alchemist? Terra's a lot like Dante. Prolonging life does not erase the inevitable. But, Terra WAS once alive prior to it's time of decay, and was a world that flourished with magic and culture. Actually, locations like Ipsen's Castle, Desert Palace and Oeilvert were originally located ON Terra, but due to the inperfect Fusion, they got fused into the surface of Gaia. Also, Terra, itself, would've looked more like a real planet and not the thing you actually seen in FFIX. I'm not sure I went into this, but it's something that is talked about in FFIX Ultimania:

Gaia, on the verge of dying, used a specifically great magical spell called "Fusion". Compare it to Meteor and Holy from FFVII, a sort of "MONUMENTAL" spell with great powers the envelopes the planet. Fusion's ability allowed Terra to scan the universe to find "new planets", worlds that have just been created. It homes in on these planets and assimilates them with Terra. But, Fusion can only occur if the "new planets" have "pure crystals".

Lesson on planet crystals: All life originates from it and returns to it. When someone dies, they take back their memories into the crystal and these experiences allow the crystal to develope more complex forms of life. "Memories" are the key to evolution. "Pure crystals" are crystals that have yet to have any life return to them. "Impure crystals" are crystals that have already begun to absorb memories of the dead.

Now, back to Terra. At that time, after cosuming MANY other planets' lifetimes just to prolong it's own existance, Terra seeked to use Fusion to continue this cycle, but Garland ran into some trouble. He could not locate any "new planets" with "pure crystals". The youngest planet he could locate was Gaia, but it already began it's path to evolution and the crystal was "impure". Garland forced the Fusion, which resulted in catastrophe. Some of Terra's locations were melded onto Gaia's surface, and in the heart of the mass Fusion, people's gene were fused into the gene of animals. Many of these fused mutations were so unstable, they died instantly. A majority of the world's population was wiped out with only a handful remaining. Terra was also sucked into Gaia's core, and exists in an alternate dimension. That is the Terra you visit in FFIX. Old Terra, probably didn't seem all too different than Gaia, in terms of looking like an actual planet."

Another quote of the same guy:
"
Garland, after waiting, could not wait any longer and forced the Fusion on Gaia. This was disasterous. It killed a majority of the people on the Gaia, and the rest were "mutated", fused with animals(the reason why there are so many animal people in the world). Only a few normal humans remained. As penalty for Garland's mistake, Terra was sucked into Gaia's core, existing in an alternate dimension.

After that, Garland had to use his backup plan, the Iifa Tree, which would act as the "Soul Divider", existing in both dimensions and relaying Terra souls into Gaia's crystal while turning Gaia souls into Mist. Mist would then give birth to monsters, and that would cause people to fight the monsters. People will die in the process, sending their souls to the Iifa Tree, which will turn them into Mist and it starts again. Eventually all of Gaia's souls would be wasted, turned into Mist. Terra's souls will be inserted into Gaia's crystal and then the Terra souls will be born into Gaia and rule the planet."

And the last one:
"
Terra was sucked inside Gaia's core. This is that actual diagram from the Japanese FFIX Ultimania site:

http://img.photobucket.com/albums/v...feFlowChart.jpg

Top lefthand corner is Gaia, and top righthand corner is Terra. Under it, in the bottom is how both function(the hole is actually the warp tunnel of the Shimmering Island). Terra is within Gaia's core and you can see that the buildings are actually within the inner layer of the core. When you actually travel to Terra, it appears to be another world, but in fact, it's within Gaia, on dimension. If you look at the sky in Terra, I believe it's when you first arrive, you see the roots of the Iifa tree stretching upward and they are wrapped around a glowing light. That's Terra's crystal. Terra's crystal is red. Red is the color of the light a crystal sheds when it's about to die. Blue is the color of the light of a healty crystal. They state that Each planet's crystal's aura is reflected by their moons. That's why one moon is blue and one is red. Terra's moon became Gaia's moon when the imperfect fusion occured, but the still shines upon it. The reason the Shimmering Island is "shimmering" is because it's the light of both crystals(Gaia and Terra) shining through the island. Now, if you look at the bottom diagram of the combined planets, you'll see that the crystal is purple. That's easy to figure out. It's showing for the presence of BOTH crystals. Iifa's roots are existing in both dimensions, as I stated. In Terra's, the roots are extracting the Terran souls from it's dying crystal. In Gaia, the roots are placing Terra's souls within Gaia's crystal and are extracting Gaia's souls, taking them to the trunk where they could be turned into Mist. That's the way to make Gaia into Terra.
"

So... Your thoughts?