Elizabeth Oliphant, James Peale
Watercolor on ivory, roughly 3 x 2 inches (7 x 5.8 cm ). Link is to Wikimedia Commons, original is in the Smithsonian American Art Museum.
In the late 18th, and through the mid 19th centuries, there was a demand for miniature portraits, both in the U.S. and in Europe. These were usually painted in watercolor or gouache on oval ivory, often in the form of pendants, and were kept as keepsakes.
Ivory seems to lend itself well to this kind of miniature water media painting, and the Smithsonian American Art Museum in Washington has a nice collection of them, accessed in drawers.
I had a chance to look through some of them on a visit to the museum a couple of years ago and I can see the appeal; many are beautifully painted, often in a delicately applied stipple technique, as is the case in this beautiful example by American artist James Peale.