In 2E D&D adamantine was described as being a shiny black color. I've always pictured it looking similar to hematite but a deeper black, like black chrome.
Doesn't the "black" adamantine come from the fact that the drow used it (in AD&D) to construct their powerful weapons that were turned to dust when exposed to sunlight?
I belive drow use Adamantite rather than Adamantine. I've always thought of what the drow use as being...well...if you know what silver looks like, and how close it is to white in color, imagine metal that has the same relation to black.
But Adamantine, on the other hand, I think of as looking almost like meteoric iron, in that it has alternating bands of lighter and darker steel-like color...like This, but shinier...After all, Adamantine is usually described as meteoric, and I think meteoric iron (as well as pattern-welded steel) look really cool, without being gaudy, like a bright green metal would be, for instance.