A farmer incidentally stumbled upon the cushion-sized "stone" when he was tilling fields in the historic Yecheng area, a 2,500-year-old city situated in what is now Linzhang County of Handan City, county's cultural relics protection department said.
The casket is 22 cm long, 19 cm wide and 9 cm high.
It is considered to be an artifact belonging to the Hinayana sect of Buddhism that prevailed in Sri Lanka, Myanmar and Thailand, He Liqun, an archaeologist with the Chinese Academy of Social Sciences told state-run Xinhua news agency.
"Such a casket containing relics of a prominent Buddhist is often enshrined in an underground palace of a Buddhist temple," he added.
Yecheng, capital of many dynasties, was once a political, economic and cultural hub in middle and lower reaches of the Yellow River.
Historical record shows that it had more than 80,000 monks and nuns in 900 temples during the Eastern Wei and Northern Qi dynasties (534-577).