Keystone

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. It is associated with the ancient white stone, serving as a symbol of victory, absolution in judgment, and the conferring of honors and rewards. 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.

Mackey