foundation stone

any of the stones composing the foundation of a building.

a cornerstone.

Origin of foundation stone

First recorded in 1645–55

The foundation-stone of a museum was laid by the prince of Wales in 1905.

Encyclopaedia Britannica, 11th Edition, Volume 4, Slice 2 |Various
By the time that they have reached it, each ball is large enough to form a foundation-stone.

Every Boy's Book: A Complete Encyclopdia of Sports and Amusements |Various
The foundation-stone is laid of a building of incalculable value;—a work of use, not of luxury.

An Account of the Bell Rock Light-House |Robert Stevenson
To lay for them the foundation-stone of a provision for old age, upon the plan of the Annuity Societies.

A Treatise on the Police of the Metropolis |Patrick Colquhoun

He says that in laying the foundation-stone no one seemed more keenly impressed than the High Commissioner himself.

South Africa and the Transvaal War, Vol. 1 (of 6) |Louis Creswicke

foundation stone

a stone laid at a ceremony to mark the foundation of a new building