HER!  I purchased this from an auction right near me in Gloucester, MA; I actually don't buy much at auctions but I became--am--obsessed with her. Look closely and there is an urn partially visible to her right, which--combined with everything else about her, really--certainly seems to indicate that our young subject is in mourning. And, painted on a box carved of ivory, perhaps she mourns a young husband lost at sea. I believe American, very late 18th/very early 19th century. 

This is in rough condition. It appears to me the portrait is painted on ivory, which shows a few splits and loss along the right edge of the urn. The portrait is backed by stiff paper, and faced with a round of cut glass, which, due to shrinkage I believe, no longer sits entirely inside the lip of the lid of the box, but rather punches out a bit, as documented. In addition, the lid no longer fits easily onto the lower half of the box without forcing, which I am loathe to do--but displays nicely set atop the bottom turned upside down. All this to say-- delicate and requiring careful handling...but a very special one.

2 7/8" d x 1 1/8". see detail photos and above paragraph re. condition.