Overstones are powerful magical objects. They are small stones, often set into rings or necklaces to avoid loss. They are also extremely durable and cannot be damaged by normal means, although certain specialized attacks from highly trained individuals can break them. Kyri Thunderhead uses such an attack to break the Endstone.
Certain individual known as "Stoners", sometimes referred to as Stoner Lords, have the ability to tap into the inherent power of each stone to create a powerful magical effect. This is often referred to as "rocking" a stone. Other stones have passive effects which affect their wielder even without rocking the stone.
They are generally used one at a time, even if the wielder possesses multiple stones, however particularly powerful Stoners such as Jon Montaine and Colindra Montaine have used more than one at a time. Jon Montaine, one of the most powerful Stoners alive, has even combined the effects of his stones, creating an entirely new ability.
The ability to use Overtones is relatively rare making Stoners extremely powerful in the world. Those without the ability cannot use the stones and are not affected by touching passive stones.
Overstones use a portion of the users own life force to power the effect, limiting how much magic they can expend at a given time. When the Stoner reaches their limit, they go into withdrawals and must spend time recovering; though a skilled stoner, who has not yet reached this limit, can tap into a portion of another person's life force to rejuvenate themselves. Certain abilities of each stone require more power than others and experienced wielders will deliberately conserve their energy to avoid reaching their limit prematurely.