what does the keystonestone actually represent in the mark degree

The keystone refers symbolically to a Masonic Covenant, carrying an import similar to that of the trowel in the Master Mason Degree 1. It is associated with the ancient white stone, serving as a symbol of victory, absolution in judgment, and the conferring of honors and rewards 1. This draws from historical usages such as the tessera gladiatoria—a reward of distinction given to the victorious—and the tessera hospitalis, which served as a token or pledge of fellowship between friends 1.

As the stone which was rejected but ultimately made the head of the corner, the keystone offers an encouraging lesson to those who experience despondency or misrepresentation 1. It teaches that although human motives may be misunderstood, attainments underrated, and reputations traduced by envious fellow mortals, there remains a divine judgment that sees beyond human error and exalts that which was wrongfully cast aside 1.

The story and symbolism of the rejected stone in the Mark Master Mason Degree were introduced into the ritual by Thomas Smith Webb 1. According to G. F. Yates, this element was not present in the original Master Mark Mason Degree from which Webb manufactured his ritual, but was brought in by him from an unknown source 1.

Scripturally, the narrative draws on the biblical passage recorded in Saint Matthew (xxi, 42) concerning "the stone which the builders rejected," which ultimately "is become the head of the corner" 1. Dr. Adam Clarke noted that this phrase originally derived from the working practices of operative masons, who would throw aside a stone that appeared improper for a particular spot, only to find later that it was the most suitable stone to serve as the head of the corner 1.