Georgia O'Keeffe primarily used charcoal for her famous 1915 abstract drawings. These works reduced forms to lines, curves, and areas of tone, making them among the earliest fully abstract examples in her career.
She produced the drawings while teaching in South Carolina and sent them to friend and former classmate Anita Pollitzer. Pollitzer showed them to Alfred Stieglitz, who exhibited some at his New York gallery in 1916.
The charcoal series marked a major break from the representational exercises common in academic art training. O'Keeffe later worked extensively in oil and watercolor, but the 1915 drawings are especially associated with her use of charcoal as a direct, spare medium.
The works are sometimes described generally as “abstract paintings,” although many of the best-known pieces from this group are drawings on paper. That distinction makes charcoal the precise answer.