Marie Curie is best known for her discovery of radium in 1898. She and her husband Pierre Curie discovered the element while studying the radioactive properties of pitchblende, an ore containing uranium. Radium was the first element to be isolated from pitchblende and it was the first element to be discovered through the use of radioactivity. Marie Curie's discovery of radium revolutionized the field of science and medicine, leading to the development of X-rays and other treatments for cancer.