Mew AquaMew Aqua (also called Mu Aqua, Aqua Crystal, and Blue Aqua) is a mysterious substance consisting of incredibly pure water in a crystaline form. It is found on Earth, and has immense healing powers and gives Mews a power boost. Mew Aqua is also good for the environment.
After the events of Tokyo Mew Mew, the Aliens left earth with a small ammount of Mew Aqua, but even that small ammount was enough to purify their planet and make it hospitable again.
Rainbow StoneRainbow Stone is a rare gem that has the ability to react with Mew Aqua and alert of its presence. It was at one point suspected to be the true form of pure Mew Aqua, but this turned out not to be the case.
In TMMR, the canon storyline is slightly altered. The ship the Aliens were on never made it all the way to their planet as it crashes, and the Mew Aqua falls back to earth.
Mew Aqua is also a key plot point in TMMR, being the focus for the antagonist's goals.
More info later.
It is thanks to the special DNA altering properties of Mu Aqua that the Mew Project was able to succeed. It's also shown to be slightly radioactive and, after long-term exposure, can cause permanent damage to the human body and in extreme cases death, much the same as any other type of radiation poisoning.
Both Mu Aqua and Rainbow Stone react by glowing slightly when in the presence of a Mew.