Short answers, pulled from the story.
Cerium is a soft, silvery-white metal with atomic number 58 and symbol Ce, classified as a lanthanide and rare-earth element. Its two main applications are in cerium(IV) oxide form: polishing materials including glass and semiconductor surfaces, and decolorizing glass. Cerium metal is also used in ferrocerium lighter flints, catalytic converters, and as a dopant in white LED phosphors.
Cerium was discovered in 1803 by Jöns Jakob Berzelius and Wilhelm Hisinger in Bastnäs, Sweden, and independently in the same year by Martin Heinrich Klaproth in Germany. Berzelius named it after the asteroid Ceres, which had been discovered two years earlier. The pure metal was not isolated until 1875, by William Francis Hillebrand.
Cerium is not rare. It makes up 68 parts per million of the Earth's crust, the same concentration as copper, and is more abundant than lead at 13 ppm or tin at 2.1 ppm. It is the most common of all the lanthanide elements, followed by neodymium, lanthanum, and praseodymium.
Cerium-doped yttrium aluminium garnet, known as Ce:YAG, emits yellow light in the 530-540 nanometer range when activated by a blue LED. The combination of the blue LED and the yellow emission from Ce:YAG produces white light, and this is the basis for most commercial white LED light sources.
During the Manhattan Project, cerium compounds were investigated at the Berkeley site as refractory materials for crucibles used in casting uranium and plutonium. The Ames Laboratory, a daughter project of the Ames project, began producing extremely pure cerium in mid-1944 and continued until August 1945.
Cerium is of low to moderate toxicity in most circumstances but carries specific hazards. It ignites spontaneously in air at 65-80 degrees Celsius, and its fire can only be extinguished with class D dry powder media because it reacts with water to produce hydrogen gas. Cerium nitrate is used medically as a burn treatment, but large doses cause cerium poisoning and methemoglobinemia. Workers exposed to cerium dust have developed itching, skin lesions, and in some cases pulmonary interstitial fibrosis.