Great Find!
It reminds me of the one I found awhile back.
This one is marked " 925 ", and above that? " GM " in the same place, on the loop, in the back...
It's also about the same size: 2- 7/8" long, and 1-1/2" wide...
My main point of showing you this one is that the first jeweler I took it to said, it look old to him, and he wasn't sure if the diamonds were real or not...He said if he had to guess, he would guess that they were real. I took it to some other jewelers, and they pretty much, just looked at it at a glance and said, Sterling silver/ Cubic Zirconium's... I still haven't taken it somewhere that I could probably trust to really find out...
A lot of them will tell you they don't put diamonds in silver, but as Jeff stated, they used to years ago...
Knowing where you found yours,, I would say, it's a good chance that it is old, and the diamonds are real...
We might want to take the to the Antiques Road Show...

Good luck & Happy Hunting~