I think Harry transformed her body into the gold ring. Then if Dumbledore were to search Harry for transmuted things (as he did), he would find the stone. Harry carefully set the ring and gem far apart, so when Dumbledore undid the transmutation on the stone it did not affect the ring. The brown color was to make the stone more attention seeking. The rest of Harry was carefully checked for magical things, but the ring itself was never checked for magic other than in combination with the stone.

Standard stage magician trick - make people look at something else.