The Opal (オパール, Opāru) is an item in the first Suikoden. The Opal is a quest item needed to recruit Esmeralda in Antei.
Suikoden
When Tir McDohl first talked to Esmeralda in the city of Antei, she jumped to the conclusion that she was going to be taken hostage. She agreed to "come quietly" and trust that Tir was acting in good faith if he brought her the Opal.
Once Tir obtained the Opal in Soniere Prison and brought it to the waiting Esmeralda, she willingly joined the Toran Liberation Army.
Trivia
- Esmeralda never actually takes the Opal from you. I guess she just wanted to look at it?