peruskivi

Noun

 * 1) cornerstone ceremonial stone set at the foundation of a building, often in the corner and inscribed with the completion year and other details
 * 2) foundation stone
 * 3)  cornerstone that which is prominent, fundamental, noteworthy, or central

Usage notes
In current Finnish practice a is a hollow block hidden in the foundation of a building, inside which a time capsule is placed in a ceremony called  and then sealed with bricks and mortar.