An energy-focusing crystal, its action analogous to a burning glass, but "infinitely" more efficient. Exactly how feldon worked, however, was (perhaps wisely) not entered into at length. The "hardest known substance in the universe", it was extremely rare, Orac mentioning traces in the asteroid belt (presumably of the Solar System) and "a few insignificant planets". The Federation was prepared to start major mining operations on planets previously considered uninhabitable to get it. Mining operations had been in progress on Mecron II for twelve years, but this apparently began before the Federation appreciated the true potential of feldon: a series of uncontrolled accidental explosions on Agrava, killing thousands of people, prompted full-scale investment. On Mecron II at least, extraction of feldon was by electromagnetic crusher: everything but the feldon was reduced to a fine dust, leaving the crystals readily removable.
Feldon was of course not only rare and useful but valuable as well, and the consignment Belkov had supposedly stored away over his twelve years on Mecron II had an estimated value of between 900 million and a billion credits. The Mecronian priests considered feldon to be a symbol of power, and one priest died whilst wearing a necklace of crystals during a lightning storm. Belkov exploited this incident to prove he had been sent by the gods to mine the crystals. His ship Orbiter was fitted with feldon panels, which could harness the energy of distant stars to control its movements.