Cast iron is a good choice but make sure your cabinet installers create a cradle for the sink since the weight might be too much to be supported by the granite and metal clips. There are also some man made materials that are nice. I would go onto Build.com and have a look at what they offer in the size and 50/50 configuration that you want.

There's always porcelain.

I'm curious why don't you like stainless for a sink?

We have Kohler 8-degree SmartDivide SS sink. My wife loves it. The divider doesn't come all the way up & is wider than most, so it's easy to rest cookie sheets & skillets on while scrubbing. Before K reno we had porcelain over cast iron. My late step-mother poured hot water in it & popped silver dollar-sized divet out of it. She wanted no part of another porcelain sink.

I don't like stainless as I have that now and it never looks clean.

My wife was concerned about that, but it hasn't an issue.