Thought I'd post this on the public forum, since it may help other folks in future...I'm only slightly embarrassed to say that I don't know enough about jewelry marks to be certain on this piece. My research was inconclusive, partly because I'd rather trust the experts here, than stuff found by googling. Anyway, I bought this artist's palette brooch in a lot with other pieces of mid-century costume jewelry, and only afterwards did I notice the mark on the back, which is a tiny "750", stamped below an illegible (to me, even with a strong magnifier) set of numbers or letters in a flattened hexagon shape--probably the maker's stamp, but who knows. When I Googled 750 marks on jewelry, it sounds like "750" can sometimes mean 18K gold. The brooch has a silver-colored edge and trim, which could I suppose be white gold? In any case, I'm not even completely certain that "750" does mean gold, since I usually only deal in costume jewelry, and haven't some across this mark before. What do you all think? I wasn't able to get a better photo of the mark with my phone, but under magnification, it definitely is marked 750. I don't currently have any method of testing for gold, and I'm not sure it would be worth spending money on a testing kit, just for this one item. I can say that the brooch is relatively heavy, for its size and thickness, and the pin-back is a bit thicker than I am used to seeing on other/costume brooches. Any thought or help would be much appreciated!