Like a deer's antler I think. Smooth with very little pitting. But I think it's also like a tooth and has nerve endings all the way through rooted to her spine, so it's very sensitive.
It feels heat and cold and can she can feel it tickle when you run your fingers along the surface. I've read some people say that it throbs slightly since it's close to her heart too. People have also said it serves as a psychic antenna too so it doesn't just sense things like we do, it also resonates with emotions and psychic energy