Woohoo

Also, where does the magic come from? Is it stored in the horn?

Shoot, deleted my response by accident. Anyway, I'd say it ultimately depends on fanon and headcanon, as I don't believe the producers ever gave a solid explanation for how magic works. Even classical unicorns have different interpretations.

One theory I've heard (and like) is that unicorn horns can influence a magical field surrounding the planet. All races, though, also have their own innate magic. Unicorns just have the ability to manipulate magic.

It could be closer to ivory.