一切經音義 22.267

22.267

卵璽（璽斯爾反鄭玄注周禮曰璽印蔡雍獨斷曰天子之璽以玉為之古者尊卑通用自秦以來唯天子得其稱璽也）。

Imperial seal shaped like an egg (luan xi; xi is pronounced si [斯] + er [爾] in fanqie. Zheng Xuan's commentary on the Zhouli says: 'Xi is a seal.' Cai Yong's Duduan says: 'The seal of the Son of Heaven is made of jade. In antiquity it was used equally by both the noble and the humble; from the Qin onwards, only the Son of Heaven was entitled to have it called xi').