I don't think so - I've never heard of Mikimoto Costume Jewelry however
when Mikimoto Cultured Pearls first arrived on the scene they were soooo much cheaper and an alernative to natural pearls. Women around the world, were ripe for a cheaper "faux" Natural Pearl. So in this sense some natural pearl stakeholders might have on ocassion termed his pearls that mimicked in almost every way natural pearls as "costume."
The quality of the pearls where of paramount importance to Milimoto. As the President of the Japan Marine Pearl Association, Kokichi Mikimoto burned 720,000 inferior-quality pearls in 1932. Reestablishing the reputation for Mikimoto cultured pearl quality.
