This is Emily Burt Bradford, grandmother and namesake of Emily Burt Holmes Marvin, from whose family papers it comes.
It is a tintype, an early form of photography. The images on tintypes, like daguerrotypes and ambrotypes, were unique photographs captured directly, meaning there was no intervening negative. The underlying support for the image is also different: tintypes were “printed” on thin pieces of iron or steel, darkened by paint, enamel or lacquer. Also called ferrotypes or melainotypes, they were more durable than other early photographic images, which were produced on glass.
This particular image has no date, but tintypes were common from the1850s into the 1930s. And this one is small. How small exactly?