Marie Curie discovered radium by carefully isolating radioactive elements in a material called pitchblende, a natural ore that contains uranium and thorium. She began this study based on the work of another scientist, Henri Becquerel, who was an early observer of radiation.
Marie Curie's discovery of radium and another element, polonium, was a long process that she undertook with her husband, Pierre. She was interested in Becquerel's work, which noted that uranium gave off rays that could penetrate objects but provided no explanation for why that occurred. She used a sensitive measurement device called a Curie electrometer to determine if other materials gave off similar rays. She discovered that thorium did.
Curie then began studying various ores to see if any of them were radioactive. Although she had varying results at first, she eventually tried pitchblende and found it was more radioactive than its uranium and thorium content would predict. She began separating the materials that make up pitchblende and testing each one individually. Eventually, she and Pierre isolated the two new elements.
Curie was able to isolate radium completely and prove that it was a new element with a unique atomic weight. She was unable to isolate polonium because it decays very rapidly. Later scientists with more sophisticated equipment and techniques were able to isolate polonium and prove that it is a separate element.