Florentine 19th Century Icon

This is 19th Century Florentine Icon   The frame is wood with goldleaf.  Small nails on each pillar are assumed to be for candles. The picture itself is a piece of slate approximately 1/4" thick.  The face of the slate is polished.   The figure of the Angel Gabriel) appears to be inlaid in pietra dura.. No markings on the item. Back of the frame is painted with light tan paint. I would appreciate any information I can get on this icon, especially its approximate value. After searching the internet high and low I concluded this is a rare piece and have not seen one other like it. Still searching for an aproximate value.