A thinner version of Homura's soul gem. It can't be printed in stainless steel as the walls of the base are too thin, however I think this thickness is closer to that of the anime.
You can have this printed in plastic and then cast it in resin with gold flakes, or try to paint it with gold paint or something. I think it would turn out just as well as the metal one.
UPDATE 03/11/2014: made base part negligibly thicker (~0.2 mm) to be printed in alumide. This model has previously had 100% success with strong and flexible plastics and their polished variants.
Stainless steel printable version
Actual gem part