Morgûl-blades were forged from celebur, a form of plutonium.
Celebur (S. "burning silver") was a form of plutonium ore. It was a silvery white or silver-grey metallic material that, when treated with certain sorcerous spells, increased its normal strong and very dangerous radiation. It was known to the Dwarves who avoided it, for it caused sickness and death even in its untreated state. It was never forged in any reputable smithy, but was used in some evil Morgûl-alloys such as mithrarian. The Morgûl-blades of the Nazgûl were forged of such an alloy; some of the evil effects of these blades were the result of the celebur within them. When Frodo was stabbed with one, the sickness it caused never left him; in the end only his leaving Middle-Earth for Valinor could cure him.