While the Stones serve as the foundation for Stonewielder magic, K'exil metal is what has allowed the Stonewielders to fully utilize their magic to the extent of its power.
It's a well-accepted fact that the Stonewielders were born when a race of beings known as the K'exil attempted to invade their dimension. While not much is known about the race, being either long-dead or busy elsewhere, happily pillaging other realms, it is known that they were a race of shapeshifters. Additionally, while the species themselves had the ability to transform, their very vehicles and weapons had the ability to transform as well. Following the battle (which was only won by several K'exil who valiantly turned against their own people), the ability to craft and program K'exil metal.
In practice, K'exil metal has the ability to change both size, mass, and shape. This allows the Stonewielders to create weapons, such as swords and bows, that can fold up into small tokens or even rings when not in use. It has also allowed for a wide range of training tools, powerful armor, and the rise of Stonejets long before humans had even begun experimenting with hot air balloons. Nearly every aspect of Stonewielder technology uses K'exil metal to function in some fashion or another.