Cobalt 247 was an invention of Dr. Frankus. He never intended any major power to possess it, as it could've been used as a terrible weapon. When it was stolen by "the other side", he disclosed it to Rudy Wells so as to restore the balance of power. BW: Fly Jaime.
Its exact nature was never specified, however it appears not to be a simple isotope of an element, as the name implies. Wells and Frankus speak of a "formula", indicating it's actually a compound.