Arguably the only one missing is watches - but there seems to be so many forums
Well, if eyeballs is the main driver, you can not go wrong with watches, imo. Slightly different crowd (and men >> women) but may pull their spouses. When those well-heeled watch-buyers see that there is true connoisseurship to gems and jewelry -- and a lot more subjectivity than even for watches (or cars) -- they may get hooked.