Vintage 18k Gold Amethyst Ring with Diamonds

Vintage cocktail ring with large amethyst (genuine?) and 8 diamonds (genuine?) in an 18k gold setting with carved accent blossoms. Approx. size 7.

There is a semi-circle with a design in it stamped near the 18k stamp. It doesn't look like a G or E (under magnification), as it may in photo 5) to indicate goldplate. But it may be another indicia that I've not seen for goldplate, or a jeweler's mark.