The Quaza Stone is a substance which the Hero Cores are made with.
Quaza is supposedly the rarest mineral in the galaxy and as Breez states in HF002: "Our core makes us who we are." Whatever is inside the Quaza Stone enables it to contain enough information to create an entire personality or even the soul of a hero. The only person who is allowed to access the room which contains these stones is Akiyama Makuro. During the 12th podcast of Hero Factory FM, DJ Mak Megahertz tours the entire Hero Factory but is not allowed in the Quaza Core chamber much to his annoyance.