Mew AquaMew Aqua (also called Mu Aqua, Aqua Crystal, and Blue Aqua) is a mysterious substance consisting of incredibly pure water in a crystalline form. It is found on Earth, has immense healing powers, and gives the 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 amount of Mew Aqua, but even that small amount was enough to purify their planet and make it hospitable again.
Mew Aqua is likely created by, or at least linked to, the Aliens, as they already know about it before anyone else does. Fragments of the stone are found throughout the series (as well as fragments thought to be a part of Mew Aqua, until they are proven not to be). In the English 4Kids dub, the Mew (Blue) Aqua is said specifically to enhance the Mews' transformational powers and be helpful to the environment, rather than simply having healing powers. However, the Mew Aqua's effect on Mew forms is only ever seen with Mew Ichigo, Mew Lettuce, and Mew Ringo.
Rainbow StoneRainbow Stone is a rare gem featured in episode 24 of the series that has the ability to react with Mew Aqua and alert others to 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.
In Mythical Mew Mew, Mew Aqua reacts to to presence of Mews by growing brighter. When in close proximity to a Mew it'll cause their Mew Marks to become warm, and if they transform the mark will begin to glow. Furthermore, Mew Aqua can unlock a second, more powerful form for Mews.
Veggie Mew Mew
In VMM, Mew Aqua is the most common to trade with. In episode 1, Ninjin is the daughter of the two parents. In the next episode, she sees Morikawa Corifurawā with the special gem.
Gem Mew Mew
In GMM, the Mew Aqua is replying on the girls’ melody.
Likeness in Fanfics
These are stones, gems, and other artifacts with similar properties as the Mew Aqua or that play a similar part in the series they are associated with.
In HMM, the "Purple Stone" is a strange round, smooth, and purple gem-like object that was buried deep into the Earth. It was found after a weapons test unearthed it, and was collected by military officials and studied in a special sector for strange phenomena. It ends up being used as a power source for the MEW project in the series, highly sought after by Velaira but protected by Gwen and Miel.
In EMM, the Rainbow Pearl is a strange, pearlesque, and rainbow-tinted artifact that fell from the sky of Arcenciel, the home planet of the Danseurs. It was kept as a decoration for a long time until it was stolen by Étoile and the Diaboliques team. Its powers were made known aboard their ship, but the pearl was lost to Earth and studied by Cadence Harper, used to create the Ensemble Mews. It is highly reactive to intense emotions like the Mew Aqua.