This Mexican sterling silver mounting is most likely dating to the mid-1990's. It is utterly simple, a scalloped sturdy frame and rope twist details. The stone is a recent addition: A glorious specimen of Montana Agate, a Dendritic quartz particular to the North Western USA.
Metal: Sterling Silver
Gem: Montana Agate
Gem Measurements: 27.4 x 19.5 mm, Oval
Ring Size: 10.75
Marks: "Mexico 925" Stamped on the inside band