Vintage IOC Signed Uncas Sterling Silver Large Solitaire 6 Prong Size 7 Ring

Stone in ring is a large 9mm across. Many of the Uncas solitaire rings are 4-pronged. It's possible this stone has 6 pronged because it is on the larger size. rise from inside of ring band to top of stone is 1/4" tall. The inside of he ring band is stamped with the Uncas logo & "Sterling" The ring has not been cleaned so it doesn't sparkle to its potential. Comes in original red velvet ring case.. Normal signs of age & gentle wear. I've checked out dozens of vintage sterling Uncas clear stone solitaire rings in order to figure out the stone's composition. Some are sold as glass, crystal, CZ & a number of other natural & simulated stones. Therefore, I've concluded that I have no clue.