Fair? Customary? I'm not sure what those two terms mean. When selling high quality, high priced gems, Two questions "who are my competitors and what do they markup." In this case; Tiffany, Cartier, Graff? and how low or high am I on the totem pole.
A couple of years ago I sold a 20+ gem. It was truly one of a kind. I bought it from a client, sold it to a dealer who sold it to a wholesale fine jeweler who consigned it to a retailer who sold it to a client. Markup brought if from the low six to the low seven figures.