www.925-1000.com

I have a little pendant that has been tested sterling and is of Navajo design. This one was bought about 33 yrs ago I believe. The mark is RB over Sterling. I will do my best to get a picture of the mark. I questioned the stone at the jewelers, it is manufactured opal according to him which then puts to question is it truly Navajo.
http://i927.photobucket.com/albums/ad11 ... endant.jpg" onclick="window.open(this.href);return false;
http://i927.photobucket.com/albums/ad11 ... ndant2.jpg" onclick="window.open(this.href);return false;

I found RB in the Southwest book but none over Sterling.

I can't identify the maker from what you say, but Navajo (and other Native silversmiths of the Southwest) commonly use synthetic opal in their jewelry.