The Cores of Proto-Matter Dragons.
They appear as round, smooth gemstone, usually blue, with a glowing red light in the center when active. When inactive they look like simple round jewels.
They are composed primarily of highly compressed Proto-Matter, in their core is the nucleus, similar in nature to a bacteria, which determines the shape of the dragon. The Pearl can absorb excess around it to produce more mass, and absorb energy. It cannot however, absorb anything else made of Proto-Matter with a different and/or opposing charge.