Mexican Silver and marble/stone sculpture

This fishy sculpture is about 5-6 inches long and 2 inches high. It has VRS MEXICO TR-50 925 stamped underneath one of it's fins. It was probably made in the 70's-80's. It looks like very tarnished silver and maybe marble or granite stone. It's a lovely piece and I'd like to know more about it, particularly how much it might be worth.