I wouldn't be suprised if it was a completely fictional material or if it was completely different to it's real world equivelant. After all, Ebony in our universe is an African wood, but in TES, it's a volcanic ore.
I always imagined it to be something like Marble, though seeing as how it's Brown coloured, it must be a different material, perhaps it's similar to marble, yet lighter? I don't know, Discuss!