noun
Urashima's casket; Pandora's box
Refers specifically to the treasure box given to Urashima Tarō in the famous Japanese folktale. When opened, it releases smoke that ages him instantly. Also used metaphorically for something that brings unexpected consequences when opened.
浦島太郎が玉手箱を開けると、白い煙が出ておじいさんになってしまった。
When Urashima Tarō opened the casket, white smoke came out and he turned into an old man.
その秘密の書類はまるで玉手箱のようで、開けたら大変なことになった。
Those secret documents were like Pandora's box; opening them caused a huge mess.
noun
closely guarded treasure; precious secret
Figurative use: something precious that one keeps hidden and rarely shows to others, like a cherished memory or a prized possession.
祖母にとって、その古い写真は玉手箱のようなものだ。
For my grandmother, that old photograph is like a closely guarded treasure.