Star's core is made of solid diamond. Ten billion trillion trillion carats. And it's only 50 light-years away, just waiting for the first interstellar salvage operation. Of course, with synthetic diamonds now as good or better than gemstone quality, this may not be as attractive as it used to be. Kind of like what happened after they figured out how to electrolyze aluminum, which used to be more valuable than silver. (The cap of the Washington monument is made of aluminum, which was then was a precious metal).