My engagement ring is a black diamond ring. It's a simple silver band with a little black stone. I didn't even know black diamonds were a thing that existed before I received it, but now I find them totally charming, so nice work, Matt! Wedding plans are slowly formulating, so I'm starting to think about wedding bands and what I would want that would complement this ring. A few ideas:
Something like this makes the most sense. It would match nicely with my current ring, is affordable, unfussy, but has a little more pizazz than a plain band.

I like the style of the above two, with the stones set directly into the band. They're also simple, very wearable, and even though my engagement ring is silver, I kind of like the idea of mixing metals with the gold band.
Speaking of mixed metals, I've lusted after this Mociun black diamond ring for a long time. But I don't know if I'd feel comfortable wearing a $3,000 ring. Especially considering I lost my engagement ring four days after receiving it (Matt found it two weeks later! In the street! Under the snow!).
And just for fun, here are some more black diamond rings, found on Etsy:
via laralewis
via beyondtherockz
via kyleannemetals
via louisagallery