Capstone
Baker's Evangelical Dictionary of Biblical Theology [1]
An architectural term denoting the top stone in a building or wall. If it caps an arch it is called a keystone. The expression is used twice in the Old Testament ( Psalm 118:22; Zechariah 4:7 ). In Zechariah 4:7 it is clearly the capstone of the temple that is in view. The meaning in Psalm 118:22 is less clear ("keystone" Jb; "capstone" Niv; "cornerstone" Nrsv). The Hebrew is translated by the Septuagint as kephalen gonias , literally, "the head of the corner." This term has made its way into the New Testament five times ( Matthew 21:42; Mark 12:10; Luke 20:17; Acts 4:11; 1 Peter 2:7 ). Psalm 118:22 evidently refers to the king who was disallowed, but then acknowledged to be the rightful ruler. Jesus makes use of this psalm in a parable referring to Israel and its rejection of him as Messiah ( Matthew 21:33-44 ,; and parallels). In Acts 4:11 Peter makes use of Psalm 118:22 to defend himself against his Jewish detractors.
In all these passages it must be acknowledged that "capstone" or "cornerstone" would make an equally good translation. It Isaiah 1 pe 2:7 that tips the scales toward "cornerstone." These three passages ( Psalm 118:22; Isaiah 8:14; 28:16 ) are used together to make the point that Christ is supreme, and two of these passages ( Isaiah 28:16; 8:14 ) clearly refer to a cornerstone.
So the Zechariah 4:7 passage is clearly "capstone" but the remainder are probably best taken as "cornerstone, " with the point being that Jesus was rejected by the ones who ought to have known him (builders = Jewish leadership), but God has exalted him to be the chief stone above all, the very head of the corner, on which all else depends.
See also Cornerstone
Webster's Dictionary [2]
(n.) A fossil echinus of the genus Cannulus; - so called from its supposed resemblance to a cap.
