Marie Curie‘s name is intimately associated with the discovery of ‘radioactivity.’ This is not accurate as that discovery belongs to Henri Becquerel. To me precise she isolated the active radioactive source within pitchblende. Starting with several tonnes of pitchblende working with he husband Pierre, she isolated one gram of pure Radium.

From the Nobel Prize website:

Her early researches, together with her husband, were often performed under difficult conditions, laboratory arrangements were poor and both had to undertake much teaching to earn a livelihood. The discovery of radioactivity by Henri Becquerel in 1896 inspired the Curies in their brilliant researches and analyses which led to the isolation of polonium, named after the country of Marie’s birth, and radium. Mme. Curie developed methods for the separation of radium from radioactive residues in sufficient quantities to allow for its characterization and the careful study of its properties, therapeutic properties in particular.