The Kingdom Crystals are seven gems that seem to be forged by the founding fathers of the Mushroom Land. It is said that these Crystals can grant powers to anyone that holds them. Because of that, Alfonso and many other villains have been trying to get their hands on them to fuel their schemes. However, Sergio and the rest of the Newsboy Legion also tries to find the Crystals to save the world.
Each Crystal can grant different powers and even be used as a power source for machines, but they're best used when one holds the seven of them.
Sergio can use them to amplify the powers that his Biometal grant him to achieve a great state of power that he himself named as Super Sergio. Shawn was revealed to be able to transform into Super Shawn as well, probably because he was implanted another Biometal.