The Reversal Jewel (反転宝 Hanten takara?) is a green broach which looks like a smily face when worn right-side up and a frowning face when worn upside-down. The broach originally belonged to Cologne, but ended up in the possession of Shampoo when Cologne allowed her to pick one of her many pieces of jewellery to keep for herself.
Despite appearing to be a simple broach, the Reversal Jewel actually has the ability to control the emotions of the wearer. When worn right-side up the wearers romantic feelings towards their chosen lover is increased and any hated the wearer feels towards this person is turned into feelings of love as well. However, when worn upside-down all feelings of love the wearer has for someone turns into pure hatred.
When Shampoo first wears the jewel, she accidentally wears it upside-down, turning her love for Ranma Saotome into pure hatred. Not knowing about the Jewel or its effects, Ranma desperately tries to get Shampoo to love him again and, due to a plan devised by Cologe, nearly ends up marrying her. However this is foiled by Mousse who makes Akane Tendo wear the Jewel right-side up so she declares her love for Ranma first before he can declare his for Shampoo.