Philosopher's stone

A philosopher's stone is an artificially created mineral substance with unique properties related to absorbing, storing, converting and generating energy. These stones also have the ability to have one soul of any person or entity stored inside them, which can then communicate with anyone who is holding or wearing the stone through a kind of telepathy.

Examples
In Yulverse, three different philosopher's stones are known.

Combustion Stone
The Combustion Stone is an octahedral red stone that seems to be made of either pyrope garnet or red spinel. It was created by an unknown alchemist and currently contains Ring. The stone has been embedded into an ornate silver ring to be worn. Ring's presence and its properties allow the wearer to create and manipulate fire.

Conversion Stone
The Conversion Stone is a brilliant-shaped, neon green stone that was found embedded inside Leo Serpentine's head after his death. It holds no soul inside it, but it allowed Leo to manipulate gravity.

Absorption Stone
The Absorption Stone is an oval, cabochon-shaped, dark blue stone created by Aemilius Dremash to serve as Grey's power source, and now holds Aemilius' soul. It has the ability to absorb any kind of energy, such as light, heat and electricity, store a large amount of it like a battery, and release it to power Grey's body.