[Casey.R]

I bought it at a thrift store for $7.99
I think it's old-ish. It's handmade, I can tell by all the dingy marks and the fact that it's not totally symmetrical. It says sterling on the inside. These days they usually say 925 and not sterling.

I'm kind of leaning towards feashwater as a guess since most of the mabe pearls seem to be, but I thought I'd ask you guys what you think.

[Valeria101]

Just a hunch: Abalone blister. Seeing one set in vintage silver is no surprise.

No expert here. I am guessing...

[CortezPearls]

My guess also leans towards Abalone: Haliotis rufescens.

Another possibility would be Tahitian Blister...most of the ones I've seen have a flat dome, like the one in the center of the photo (left to right: Pteria penguin, Pinctada margaritifera, P. maxima).
Your photo needs a close-up in order to catch more of the surface and the colors present.

[Casey.R]

Mine has that same type of dimple.

[Ingykiz]

Cool ring.
I need to go to thrift shops more often. My friend got an antique pin with real seed pearls for one dollar at a church bazaar. I am never that lucky.