Georgia O'Keeffe used charcoal for the abstract works she made in 1915.
During 1915, O'Keeffe produced a series of radically simplified charcoal drawings. Rather than copying recognizable subjects, she explored line, shape, rhythm, and tonal contrast. The series is widely regarded as a breakthrough because it made nonrepresentational art central to her practice.
She sent some of these drawings to Anita Pollitzer, who showed them to Alfred Stieglitz in New York. Stieglitz exhibited them at 291 in 1916. This event helped establish O'Keeffe as an important emerging modernist, even though she was still teaching and had not yet become widely known.
O'Keeffe later became especially associated with oil paintings of flowers, bones, buildings, and landscapes. That later reputation can make viewers overlook the importance of charcoal in her early abstract development. The question concerns the 1915 series, not her best-known later medium.