Vintage 18K White Gold Filagree Engraved Gemstone Diamond Ring Size 5.25

Beautiful vintage find! Set securely in the center of the is a large marquise shape gemstone. I will 100% honest and tell you that I am not sure what the stone is. It appears to have been all black at some point with more then half of it faded away. But the black that does remain is not just on the surface, it is inside the stone. So I'm not sure what to call it! The stone measures 18 x 9mm and is held in with four prongs. It could easily be replace since all prongs are in good condition. There is one white gold deal in the center of the stone with a diamond set in the middle. Although the stone may not be in its best original shape, the mounting is in very good condition. It has great scroll filagree cutwork all around the stone and down both sides of the band. It measures just under 7/8 across the center and the stone sits 5/16 off of your finger. The band has a shiny high polished finish that would be easy to size up or down (locally). The setting is solid white gold, stamped 18k