Refers to the base stone of a building or, figuratively, the fundamental basis of something. Common in architectural, historical, and metaphorical contexts.
The foundation stone of this temple was laid eight hundred years ago.
The foundation stones of this temple are over a thousand years old.
教育は国の発展の礎石である。
Education is the cornerstone of a nation's development.
His research became the cornerstone of modern medicine.
土台 refers concretely to the base structure supporting a building, whereas 礎石 carries a stronger symbolic sense of being the essential origin of something.
Used for foundation stones (礎石) set as the base of a building.
Compound of 礎 (foundation, cornerstone) and 石 (stone). The word directly describes a stone used as a foundation, and its figurative use is a natural extension.