The capstone, “the top stone on a wall or parapet around a flat-roofed building,” was a stumbling block to some. “Many Jews similarly tripped over Jesus’ identity and plunged to their destruction. Likewise, a capstone could fall on someone below and crush him or her. These are allusions to Isaiah 8:14–15 and Daniel 2:35, 44–45. Jesus was a dangerous person, as well as God’s chosen representative and the occupier of God’s choice position in His building, Israel. Jesus was claiming to be the Judge; He would crush those on whom He fell” (Thomas L. Constable, Notes on Matthew [Sonic Light, 2022], 359, https://www.planobiblechapel.org/tcon/notes/html/nt/matthew/matthew.htm).
Here you can see an image of a capstone: https://www.britannica.com/topic/dolmen/images-videos.