Akrogōniaios (G204) means the cornerstone at the extremity — the foundational stone placed at the corner of a building that determines the alignment of all walls. In the NT it is applied to Christ as the chief cornerstone of the church, citing Psalm 118:22 and Isaiah 28:16. The stone rejected by the builders becomes the most important stone.
The akrogōniaios is the architectural key to the whole building. In Christ, Jews and Gentiles are joined into one building (Ephesians 2:20–22) — the two walls of humanity are aligned and united in him. Peter's use (1 Peter 2:6) presents Christ as both chosen and precious. Those who believe are not put to shame; those who reject him find the cornerstone becomes a stone of stumbling.