Black Materia (Final Fantasy VII)

The Black Materia is a mysterious and incredibly powerful Materia, as well as one of the oldest, being around in the time of JENOVA and the Ancients. It and its counterpart White Materia are the largest and most unique of all Materia on the Planet, being only one of each. It is used to summon the "Ultimate Destructive Magic," Meteor, with the power to destroy the entire Planet. Few knew truly of its existence, let alone how to use it.

Story
After Sephiroth fell into the Lifestream and spent a year in half-dead isolation in the Great Northern Crater, he began his plan for ultimate domination of the Planet. Finding the legendary Black Materia, he would absorb the Lifestream energy that rushed to the wound to the Planet, and use the power of the Planet's life to find a new planet on which he would be the dominant force, just as JENOVA had tried to do.

Upon learning this plan, AVALANCHE rushed to the Temple of the Ancients where the Black Materia lay resting to try to stop Sephiroth from obtaining it. Arriving there, they discovered that the temple was really the Materia itself transfigured by the Cetra. It is assumed that the Cetra who accomplished this were the few last survivors after sealing Jenova, as the murals in the Golden Hall depict the story of Jenova as if to warn those wishing to obtain the powerful stone. It could only be shrunk back into Materia by solving its puzzles from the inside, at which time, the person attempting to obtain the Materia would be crushed inside the Black Materia, as a final defense against any evil trying to use it.

However, Cait Sith offered to use his mechanical body to solve the puzzles. Unlocking the Black Materia, he was crushed inside. However, a second Cait Sith promptly arrived to take the first's place. Cloud picks up the Black Materia, but Sephiroth is able to bend his will through the JENOVA cells in Cloud's body, and forces Cloud to give it to him, undoing the entire process.

After defeating Jenova DEATH in Knowlespole, the party reclaims the Black Materia. Cloud here can entrust it to either Barret Wallace or Nanaki, although no matter which he gives it to, the ending is still the same. Sephiroth's illusion spell will influence them to give it to the mentally unstable Cloud, who then delivers it to Sephiroth. Sephiroth uses it to summon Meteor before the WEAPONs arrive, and the Materia falls into the center of the Planet, never to be heard of again.

As Meteor falls, even Holy isn't able to stop it, and the Lifestream itself bursts out of the Planet, and pushed it back out, starting Sephiroth's plan B: Geostigma

Appearance
As it's name suggests, the Black Materia is purple-black in color. Unlike most Materia, that are rather small, it appears to be about grapefruit-sized, perhaps as a way of representing its immense power. However, in Final Fantasy VII: Advent Children, all Materia appear to be the same size.

In its transfigured form, the Temple of the Ancients, it is in the shape of a step pyramid, with its entrance at the top of a long staircase, like the pyramids of the Aztecs. Within it are several passages, some of which have carved murals telling the story of the White and Black Materia, and the coming of JENOVA.

Other Appearances
In Final Fantasy Tactics, there is an artefact that can be obtained from an Errand called Black Materia.