Vintage Silver Ring Brown Stones

Vintage silver ring with brown stone centrepiece from Mexico. Not entirely sure what it is... Blue John? Agate? Amber? Let me know if you recognize it. Stamped 925. Size 4.75. In very good pre-owned condition, with possibly a hairline crack in one side? It's not loose at all and kind of adds to the design. Want more info? Tune in to my next Posh live show :)