I'm pretty sure magical weaponskills are al lthe ones that are anything but white for ability type in the list.
It may be that the first hit of a magical weaponskill is the magic hit, and the rest are normal, therefor if you kill a mob with a multi hit magic ws and the first hit was death shot, then its amber light, if it was the second that finished it off its ruby.
That's what I'm leaning towards, I can't really tell because all my non physical weaponskills are confirmed magic, except shadowstitch I'm not sure on, but I'm pretty sure thats magic too
