Very Unusual Art Glass Rectangular Domed Brooch in Silver Tone Metal

This is a simple yet elegant rectangular pin with a lovely mottled blue stone that is exquisite. The stone is surrounded by a silver plated frame with a ball edging. The pin is very old, I would say the early 20th century. It is not marked. The silver plating is coming off the back of the pin but doesn't detract from the beauty of the pin.