Gadolin became famous for his description of the first rare-earth element, yttrium. In 1792 Gadolin received a sample of black, heavy mineral found in a quarry in the Swedish village Ytterby near Stockholm by Carl Axel Arrhenius. By careful experiments, Gadolin determined that approximately 38% of the sample was a previously unknown "earth", an oxide which was later named yttria. Yttria, or yttrium oxide, was the first known rare-earth metal compound — at that time, it was not yet regarded as an element in the modern sense. His work was published in 1794.

The mineral that Gadolin examined was named gadolinite in 1800. The element gadolinium and its oxide gadolinia were named after Gadolin by its discoverers.

In an earlier paper in 1788 Gadolin showed that the same element can show several oxidation states, in his case Sn(II) and Sn(IV) 'by combining itself with larger or smaller amounts of the calcinating substance'. He vividly described the disproportionation reaction:

സംഭാവനകൾ
പിൽക്കാലജീവിതം
ഇട്രിയം, ആദ്യ റെയർ എർത്ത് മൂലകം
രസതന്ത്രത്തിലെ നേട്ടങ്ങൾ