Georgia O'Keeffe used charcoal for the abstract drawings first shown by Alfred Stieglitz in 1916.
O'Keeffe made a group of abstract charcoal drawings in 1915 while teaching in South Carolina. She sent some of them to her friend Anita Pollitzer, who showed them to Alfred Stieglitz in New York without O'Keeffe's prior permission.
Stieglitz was impressed and exhibited the drawings at his gallery 291 in 1916. The event marked O'Keeffe's first solo exhibition and helped establish her as a significant emerging modernist. Her early abstractions used simplified lines and forms rather than recognizable scenes.
O'Keeffe later worked in oil, watercolor, pastel, pencil, and other media. That range can make the first-exhibition story easy to misremember. The works that triggered Stieglitz's attention were specifically charcoal drawings, not oil paintings or watercolors.