The Phantom Ruby (ファントムルビー,Fantomurubī?), also known as the mysterious gemstone, is an object that appears in Sonic Mania. It is a gemstone of incredible power that was fought over by Sonic the Hedgehog, Dr. Eggman, and their respective parties.
The Phantom Ruby is a magenta icosahedron ruby. While different from the Chaos Emeralds, the Phantom Ruby is still remarkably powerful and has been theorized to be a source of unspeakable power. When channeling its power, the Heavy King became powerful enough to bypass the invulnerability of a Super State. Its power is also so great that it affected the Master Emerald's ability to keep Angel Island afloat, causing it to fall back into the ocean once more.
The Phantom Ruby's exact powers are unspecified, although it is capable of warping the fabric of space-time by emitting a wave of energy that temporarily distorts the surroundings. This power is primarily used for teleportation of the user and others, and can 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.
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 (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.
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. The fate of the Phantom Ruby remains unknown.
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. 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.