连城之璧 liánchéngzhībì
[a very valuable and rare thing] 价值连城的美玉。《史记·廉颇蔺相如列传》:“赵惠文王时,得楚和氏璧。秦昭王闻之,使人遗赵王书,愿以十五城请易璧。”后因以“连城之璧”比喻极珍贵的东西