Trying to identify Mexican makers mark RMM on ring

[user701]

I have looked online without much success trying to ID the maker of a ring I found today clearing through some things at home.

Curiosity has got the better of me! The makers mark is RMM

It is marked sterling and has the eagle mark with a 3 within, which if I remember correctly, is for Taxco.

I am not sure what the stone composition is, some form of agate.

Any help in with the maker would be appreciated and if anyone knows that the stone is, would also be appreciated.