925 gold plated WORN/tarnished ring with amethyst? Size 9 VINTAGE

The inside of the ring has a 925 marking It is/was gold toned so likely gold plated but the gold plating has REALLY worn off. See photos. Stone doesn’t seem loose or damaged, just dirty. And when I test it with my diamond selector ii it does increase - see last few photos for what I mean. So it might be amethyst? Photos for details and measurements Approx size 9 Priced accordingly