Red Stone of Aja

The Red Stone of Aja (エイジャの赤石, Eija no sekiseki) is the perfect gemstone sought by the Pillar Men in order to greatly focus sunlight. In conjunction with the stone mask, the stone would enable them to withstand sunlight.

History
About 2,000 years ago, this perfect stone was in possession of an unnamed Roman Emperor. Kars, Esidisi, and Wamuu pursued it, but were defeated and imprisoned by the Hamon warriors of the time.

The reason the Stone was not destroyed to prevent the Pillar Men from using it was due to a legend passed down among Hamon users, warning that destroying the Stone would make defeating the Pillar Men even more difficult. (This is later proven to be true when it is shown that the Stone amplifies sunlight to a beam capable of burning human skin, and when Cars's Hamon in his ultimate form, upon being amplified by the Red Stone of Aja, causes an overflow of energy enough to force a volcano to blast him into outer space).

The Stone was passed down among Hamon users, eventually ending up in the possession of Lisa Lisa during Part 2.

Battle Tendency
During the final battles of Part 2, Kars was able to use the Red Stone of Aja to become the ultimate life form. However, just as he is about to deal the finishing blow to Joseph Joestar, Joseph is prompted to hold up the Stone, amplifying Cars's Hamon several times and causing the volcano beneath them to erupt and blast Cars into space, where he remains unable to return to Earth and eventually stops thinking and desires death. The whereabouts of the Stone afterwards is unknown.

Trivia

 * Aja literally pronounces as "Asia".
 * The Stone's name comes from the Steely Dan Album, Aja.
 * Kaguya Houraisan from Touhou Project has a Spell Card named New Impossible Request "Red Stone of Aja" referencing it.
 * In the 18th chapter of Part VIII: JoJolion, a gem seen on the little girl's compact has a similar appearance to the Stone.