Refers to the base stone of a building or, figuratively, the fundamental basis of something. Common in architectural, historical, and metaphorical contexts.
The foundation stones of this temple are over a thousand years old.
Education is the cornerstone of a nation's development.
The foundation stone of this temple was laid eight hundred years ago.
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.