Phantom Ruby

The Phantom Ruby (ファントムルビー), also known as the mysterious gemstone, is a recurring object in the Sonic the Hedgehog series. It is an interdimensional gemstone of incredible power that was fought over by Sonic the Hedgehog, Dr. Eggman, and their respective parties. It plays a central role in Sonic Mania and its follow-up game Sonic Forces.

Description
The Phantom Ruby is a magenta icosahedron ruby. While different from the Chaos Emeralds, it is still remarkably powerful and has been theorized to be a source of unspeakable power. When channeling its power, the Phantom King became powerful enough to bypass a Super State's invulnerability. Similarly, the ruby itself can also restrain the Chaos Emeralds and outright depower a Super State. Its power is as well so great that it could affect the Master Emerald's ability to keep Angel Island afloat, causing the island to fall back into the ocean. The Phantom Ruby's abilities are said to outclass even those of the Master Emerald's.

The Phantom Ruby's exact powers are based on virtual reality control. It takes control of people's visual and depth perception to feed false information to the brain creating a new reality for them. This includes empowering and altering beings (like the Hard-Boiled Heavies and Infinite) or creating illusory places and entities that feel real. Its powers can also warp the fabric of space-time, allowing it to create pocket dimensions such as Egg Reverie Zone and Null Space. When used alongside the Chaos Emeralds' time-space powers, it transported Classic Sonic and itself to another dimension.

When touched, the Phantom Ruby, as a default, seems to generate whatever the person who touched it most desires, although these only last as long as it is being physically touched by the person. For example, when Infinite accidentally touched the gem, he was struck by images of a destroyed world of chaos, based on his own violent desire for change.

When used, the Phantom Ruby creates a by-product known as the Ruby Mist, a mist-like substance that distorts space-time within it.

Sonic Mania
In Sonic Mania, the Phantom Ruby appeared on Angel Island after a dimensional breach occurred in the atmosphere, its effects causing Angel Island to fall into the sea again. Its unique energy signature was soon detected by Sonic the Hedgehog and Miles "Tails" Prower who went to investigate it. Also detecting the gemstone was Dr. Eggman, who sent his team of elite Egg-Robos, the Hard Boiled Heavies, to retrieve it after theorizing its potential.

When Sonic and Tails arrived on Angel Island, the Hard Boiled Heavies were excavating the Phantom Ruby from the ground. As they did so, the gem warped space-time around them and catapulted Sonic, Tails, the Hard Boiled Heavies, and Knuckles the Echidna (who was relaxing nearby) to Green Hill Zone. In the process, the Phantom Ruby empowered the Hard Boiled Heavies, transforming them into more powerful Egg-Robos with wills of their own.

The Hard Boiled Heavies promptly escaped with the gem which was soon taken by Dr. Eggman, whose usage of the gem sent Sonic and co. away when they caught up to him. When the trio later cornered Eggman in Press Garden Zone, the doctor used the ruby to sent them to Little Planet.

Eggman ultimately made the Phantom Ruby the power source of his Titanic Monarch, its power distorting space within Eggman's creation. When Sonic and co. faced Eggman, the doctor used the gem to power a battlesuit. Upon Eggman's defeat, the Phantom Ruby pulled the doctor and Sonic into a strange space where Eggman and the traitorous Phantom King fought over the gemstone, only for Super Sonic to defeat them both. In the aftermath, the Phantom Ruby reacted to Sonic's seven Chaos Emeralds, causing Sonic and the ruby to be pulled through a spatial warp to another dimension.

Sonic Forces
Several month prior to the events of Sonic Forces, the Phantom Ruby (though it had no name at the time) ended up near the Arsenal Pyramid, where it was found by Dr. Eggman after his base's energy sensors detected it. Upon touching the Ruby, Eggman had a brief vision of Eggmanland. Intrigued by the Ruby, Eggman researched it, eventually coining the name "Phantom Ruby". At that very moment however, his base was attacked by Jackal Squad, led by the ultimate mercenary. During the attack, the Phantom Ruby manifested a troop of Egg Pawns in response to Eggman's desires. The ultimate mercenary then attempted to kill Eggman, but struck the Ruby instead, causing an illusion of a ruined world based on his own violent desire to appear. Eggman, having seen the vision as well, was impressed with the mercenary and convinced him to join his empire and achieve their mutual goals.

While mercenaries protected him, Eggman analyzed the Phantom Ruby, hoping to use its powers in his bid to take over the world. Eggman constructed several bases to conduct these experiments and created countless "Phantom Ruby prototypes", with most having one or several fatal flaws that proved too risky to use effectively. Eventually, Eggman finalized a Phantom Ruby prototype. That prototype was given to the ultimate mercenary, who, after a humiliating beatdown at the hands of Shadow, casted away his previous identity and assimilated with the prototype after a number of experiments, becoming Infinite. Under the direction of Eggman, he sought to destroy the world and remake it with the Phantom Ruby.

For his first field test of the original Phantom Ruby, Eggman went to Angel Island and used the Ruby to create an illusion of the Pachacamac's village and make Knuckles the Echidna and Silver the Hedgehog fight a replica Chaos. Although Knuckles and Silver overcame Chaos, it filled Eggman with glee that the Phantom Ruby worked so effectively. Eggman and Infinite then put their plans to action by creating replicas of Shadow, Metal Sonic, Zavok and Chaos, defeating Sonic the Hedgehog, and conquering 99% of the planet.

After conquering the world, Eggman and Infinite went about destroying all the Phantom Ruby prototypes so they would not fall into the wrong hands and potentially throw a wrench in their plans. During Infinite's battle with Silver in Mystic Jungle however, Silver knocked one of the prototypes onto the ground, which a rookie member of the Resistance later discovered. The Resistance later destroyed the Death Egg, which weakened the Phantom Ruby, but Eggman had anticipated that they would target the Ruby's power sources and built a backup power supply underneath Metropolis, with it regaining enough power to seal away Sonic into Null Space and ultimately allow him to create an artificial sun that would crash down onto the planet.

After the rookie foiled that plan by using the Phantom Ruby prototype they had found to neutralize the sun though, the Ruby was weakened again, but recharged during Sonic's battle with Infinite. The heroes then destroyed what was assumed to be the generator holding the Phantom Ruby, though it was later revealed to be powering Eggman's newest Death Egg Robot and use to create thousands of replicas of Shadow, Chaos, Zavok, Infinite, and Metal Sonic. Ultimately, the teamwork of both Sonic counterparts and the rookie ultimately neutralized it for good.

Sonic Forces: Speed Battle
In Sonic Forces: Speed Battle, at the end of each course, the player can see a gemstone that is referred to as the "Phantom Ruby Prototype". This gem also closely resembles the icosahedral gem on Infinite's chest.

Trivia

 * The sound effect made by the Phantom Ruby when it distorts space-time is the same as the one Infinite's cube attack makes in the Sonic Forces E3 Trailer. Extracted sound data from Sonic Mania reveals that the sound file's names for the sound effect were "RedCube_R" and "RedCube_L", further hinting at a connection between the Phantom Ruby and Infinite. Similarly, when it reacted to the Chaos Emeralds after the final boss in Sonic Mania, it briefly exhibited a cube-based distortion aura like Infinite's in the "Enter Infinite" trailer for Sonic Forces.
 * Sonic also poses the exact same way as in the reveal trailer for Sonic Forces after the Phantom Ruby warps him somewhere. This has led many fans to believe that the ending of Sonic Mania may tie into Sonic Forces, which was later confirmed by Takashi Iizuka during the Tokyo Game Show 2017.
 * Prior to officially being named by Dr. Eggman, Cubot proceeded to suggest names for it such as the Spooky Sapphire, Anarchy Beryl, and Precious Stone, with Orbot rejecting the names for various reasons (specifically, that it didn't shine blue, that it was too derivative, and it was from the wrong game, respectively). The latter two names were a reference to the Anarchy Beryls of anti-Mobius from the pre-Genesis Wave continuity of Archie Comics' Sonic storyline and the Precioustone from Sonic Shuffle, respectively.