Helena's ring
I tried the shiral effect at first. It looked OK but not great, since I could duplicate the color of an amber stone but not the transparency, so I tried going with Jerusha's idea and added just the tiniest bit of a light blue to some pearlized white paint. It looked right when it was wet, but unfortunately it turned out looking silver instead of milky-blue when it dried. That still works, though, as I can see Helena using a polished mirror ring for scrying, and a small disk of polished metal would probably be more affordable for the purpose than shiral.
Helena's ring
I tried the shiral effect at first. It looked OK but not great, since I could duplicate the color of an amber stone but not the transparency, so I tried going with Jerusha's idea and added just the tiniest bit of a light blue to some pearlized white paint. It looked right when it was wet, but unfortunately it turned out looking silver instead of milky-blue when it dried. That still works, though, as I can see Helena using a polished mirror ring for scrying, and a small disk of polished metal would probably be more affordable for the purpose than shiral.