金石 literally means 'metal and stone', but in classical Chinese it refers to bronze vessels and stone tablets used for inscriptions — the primary sources for ancient Chinese epigraphy. The term also metaphorically represents firmness and durability (as in the idiom 金石之交 'a friendship as firm as metal and stone'). In modern contexts, 金石 can also refer to gold and precious stones, or to weapons. You'll most often encounter it in historical or literary texts.