Cameo Portrait

Various web searches have yet to find anything quite like this piece. Perhaps me not knowing precisely how to describe it is the reason. I've been calling it an "antique cameo portrait," but hopefully you can elucidate me as to it's proper name.

The piece has been in my family for many years. I remember as a child admiring it on the mantlepiece. I was always told it was my great grandmother, who lived close to 100 years. My mother & grandmother both into their 80's puts this piece somewhere around 1760-1780 as best as I can determine.

The red velvet facia around the portrait was a deep red wine color when I was a child, but has faded now. The frame appears to be made of brass, which probably would have appeared as gold when polished back then, the present patina has been left unaltered. The glass is convex & in perfect condition.

I'm not sure if the portrait is a photo which has been colorized by hand painting, or if the picture is entirely a painting. There's a tiny signature vertically up the right side above her shoulder, the first letters are illegible even under a powerful magnification, but the last name appears to be Carriera. This name isn't one I recall as being connected with my family tree.

Any information you could provide would be appreciated immeasurably.

Thank You

PS - I have more detailed photos if you require them.