I don't have a material but as a long time member of www.coffeegeek.com...

Most freshly roasted coffee has got patches of slightly different colours in it.  Slightly slick darker oily patches over a deep brown.  The other thing is that the groove in the middle quite often has a light streak that runs down the middle (it's where the chaff created during roasting doesn't come off.

You'll want to have a bump map on the material as the surface won't be smooth.  One thing on the model is that the bean I've got in front of me is convex at the top while your model looks like it's concave.  A green coffee bean would be concave but during the roast the carbon-dioxide in the bean expands so the whole thing looks fatter and plumper.

I might have a play with materials for you if I get a chance.