gigantic stone; megalith; massive boulder
Formal or literary term for a very large stone, often used in archaeology or dramatic description.
その遺跡には、古代の巨石がいくつも残っている。
Several ancient megaliths remain at the ruins.
山道に巨大な巨石が落ちていて、通れなかった。
A massive boulder had fallen onto the mountain path, blocking the way.
巨石 is a more technical or archaeological term for a huge rock or megalith, whereas 巌 carries a classical, aesthetic nuance.
Compound of 巨 (huge) and 石 (stone). The reading きょせき is the on'yomi of both characters.