The Stones are the foundation of Stonewielder magic. Without them, Stonewielders would simply be ordinary humans. In short, the Stones are interdimensional minerals that allow for extraordinary feats to be performed by whoever is holding them. The Stones can take many different forms, allowing for a wide range of powers and abilities.
There are two main types of Stones. The first, and least visible, are the microscopic deposits of Stones inside Stonewielder bodies. These tiny Stones have been infused into their bones, their blood, becoming an integral part of their biology. These are what allow Stonewielders to use Stones in the first place, and what prevents ordinary humans from interacting with any larger Stones that may be found around the world.
The second type are macroscopic Stones. These, simply put, are Stones that you could hold in your hand. The larger the Stone, the more power it imparts, generally speaking. Once again, only Stonewielders can use macroscopic Stones, an ordinary human would simply see a crystal.
That, of course, brings us to what the Stones look like. Each Stone will have a specific color, with that color denoting what power is granted by that Stone. For example, red Stones allow the user to emit enormous blasts of energy, while purple Stones allow for extraordinary speed. Skilled Stonewielders can use multiple Stones at the same time, chaining together multiple abilities into more powerful combinations.