One of three similar rings I bought years ago. The stone is likely a translucent carnelian and is set in sterling.

I couldn't find a sterling stamp so I tested it with dilute acid. The result was a gunmetal appearance, no green and no fumes that would indicate an overlay only. 

This ring is size 5 and the center is 1 1/8" by 7/8." The stone is about 7/8" by 5/8."