“The stone the builders rejected”
The stone the builders rejected has become the cornerstone. (Psalm 118:22 BSB)
What it sounds like today
A minor construction detail about a leftover stone.
The cultural key
In ancient building, the cornerstone (literally 'head of the corner') was the single most important stone in the structure: the large, carefully squared block laid first at the corner to set the lines and angles of the whole wall, or the capstone that locked an arch together. Everything was aligned to it. For masons to 'reject' a stone meant judging it flawed and throwing it on the discard pile. The proverb's shock is the reversal: the very stone the professional builders threw away as useless turns out to be the load-bearing cornerstone the whole building depends on. Israel sang it of a nation, or a king, despised by the great powers yet chosen by God as the foundation. The New Testament writers seized on it again and again for Jesus, the rejected one made the foundation of everything.
Recovered meaning
The cornerstone was the crucial stone that set a building's lines and carried its weight, so a rejected stone becoming it is a total reversal: what the experts discarded proves to be the foundation. It became the New Testament's favorite image for Christ.