Georgia O'Keeffe used charcoal for many of her early abstract works on paper.
In 1915, O'Keeffe made a groundbreaking group of abstract charcoal drawings while teaching in South Carolina. The works used flowing lines, geometric arrangements, and tonal contrasts to explore visual ideas without directly depicting recognizable objects. They are among the earliest purely abstract works produced by an American artist.
O'Keeffe sent some of these drawings to her friend Anita Pollitzer, who showed them to Alfred Stieglitz in New York. Stieglitz exhibited them at his gallery 291, helping launch O'Keeffe's professional association with the New York art world. The drawings' charcoal medium was central to their direct, experimental appearance.
O'Keeffe later became famous for oil paintings, especially enlarged flowers and New Mexico subjects. That later fame can obscure the importance of the charcoal abstractions. The question concerns the early works on paper, not the medium most associated with her mature color paintings.