This is a cabinet photo (a little more than 4 x 6 inches, including the mounting) of woman wearing a dark short sleeved dress. It was taken by F. B. Merker of Belleville, Illinois. There is no other identifying information.
My best guess is that this is from the 1880s or 1890s, and it was unusual for a woman (or a man for that matter) to display bare arms in a formal portrait. Also, I originally thought that was a boa draped over her shoulder, but it isn't - it's her hair, probably in a long pony tail. Most women her age, in that era, would not have worn their hair down.
Maybe she was an actress (you know how they are). Maybe she was ahead of her time stylistically.
Or maybe it was a fad, or maybe it was well within the moral conventions of the time, or perhaps it was the latest and greatest from the fashion houses of Paris. I honestly have no idea.
Unique photos are hard to find. If it were easy to find them, they would not be unique. I suppose every photo is unique in some way, in fact I know they are, but some are more unique than others. This woman's bare arms and long hair draped over her shoulder sets this photo apart.
(Old joke: how do you find a unique photo? You neek up on it.)