Unusual Blister Pearl Ring in Sterling - Vintage

I believe this to be a natural blister pearl ring set in a sterling mounting. Oval shape with irregular point in center. Very colorful greens and coral colored mostly. Set in a plain sterling mounting. Measures a size 4, so probably a pinkie ring for most. Good vintage condition. A natural blister pearl forms naturally as a whole pearl within a pearl sac in a mollusk. The natural pearl breaks through the sac and eventually finds its way between the shell and mantle. Layers of calcium carbonate are secreted over the pearl and attach it to the shell. The completely merged entity is now by definition a natural blister pearl. As the mollusk grows older, the thickness of the enveloping layers increases. Unlike whole pearls, these are removed from their shells through cutting and grinding in order to be suitable for use in jewelry. From my mom’s jewelry box. J-1500