Nothing beats a childhood favorite! And lucky for us, Usagi's magical items are not only subtle in origin, but also make for great day-to-day jewelry.
This is Usagi Tsukino's season 3 (anime) transformation item: the Cosmic Heart Compact housing her Silver Crystal (Ginzuishou). It helps her transform into Sailor Moon / Sailormoon throughout the "Infinity" arc.
The item can be worn as a pendant, or it can be attached to a ribbon and worn for cosplay purposes. As can be seen in the Shapeways 3D model view, the hook is hidden in the back.
Moon Cosmic Power, Make Up!