If you want to stay Somerville local, you may want to check Jade Moran jewelry on Highland Ave. Right outside of Davis really. I have a friend who's wedding band comes from her store and she really loves it.
I second the recommendation for Jade, as she's fantastic. I'm not sure if she does tungsten off the top of my head. I think when we were looking at our wedding jewelry, she couldn't do titanium because she didn't have a caster who could do it, but it's worth asking, particularly if you want custom jewelry done.
Gideon Weisz (http://www.gideonweisz.com/) operates out of Moran's store; I don't know what metals he uses, but he designs a lot of cool science-themed rings (I saw "tungsten" and immediately thought "science," but since I know nothing about wedding rings apologies if I'm way off-base!)
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