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 a 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. In fact, by comparison, the Phantom Ruby's abilities outclass even those of the Master Emerald's.

The Phantom Ruby's exact powers are based on virtual reality control, although it is capable of warping the fabric of space-time by emitting a wave of energy that temporarily distorts the surroundings. This power allows the gem to warp targets across space and seemingly influence gravity. The ruby’s power can also be harnessed to create energy constructs and open portals in space-time. The Phantom Ruby can also empower and alter certain beings, like transforming the Hard Boiled Heavies into new forms with unique powers far beyond their original capabilities and free will. It takes control of people's visual and depth perception to feed false information to the brain creating a new reality for them. Owing to this, it can also create constructs of whoever touched it desires most, as evidenced by Dr. Eggman and the Jackal Squad's leader's visions of Eggmanland and a desolate wasteland, respectively, although these only last as long as it is being physically touched by the person. Similarly, it can also generate things a user verbally desires, as evidenced by it unleashing Egg Pawns after Eggman expressed regret in his failing to maintain the base's defenses during his research.

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 into a spatial warp.

Sonic Forces
In Sonic Forces, the Phantom Ruby ultimately made its way to the Arsenal Pyramid, the hideout of Dr. Eggman from a parallel dimension. Eggman then proceeded to investigate the Ruby, only to be met with a vision of Eggmanland, which disappeared immediately after dropping the gemstone. Intrigued by the Ruby's properties, Eggman proceeded to research it, eventually coining the name Phantom Ruby. However, shortly after, his base was attacked by Jackal Squad, led by the Ultimate Mercenary. Eggman initially was outraged that he wasted resources on researching the ruby than building up his army, although the Ruby proceeded to materialize Egg Pawns out of thin air. The leader of the Jackal Squad then attempted to kill Eggman, but the touch of his sword on the ruby proceeded to show him a desolate wasteland. Eggman, seeing the vision as well, was impressed with the leader, suggesting they form an alliance to achieve their mutual goals.

At some point later, he used its power to distort space-time on Angel Island, sending Knuckles the Echidna and Silver the Hedgehog back to a time where Pachacamac's village still existed. There, the duo fought Chaos and defeated him, putting an end to the Phantom Ruby's effects. Regardless, it filled Eggman with glee that the Phantom Ruby worked so effectively.

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.