what is the historical origin of the immovable jewels in craft masonry and how do preston and mackey explain their symbolic meaning

Early eighteenth-century lodge lectures list the immovable jewels as the Tarsel (or Trasel) Board, the Rough Ashlar, and the Broached Thurnel 1. By 1802, W. Finch recorded that these jewels were designated as immovable because they lay open in the lodge for every brother to moralize upon 2.

Albert Mackey identified the early "Broached Thurnel" with the Perfect Ashlar 1. He explained the symbolic contrast between the Rough Ashlar and the Perfect Ashlar not as a comparison between different men, but as an emblem of a single individual's self-development—representing the difference between a man's initial, uncultivated state and his later refined character 3. Mackey also noted that while modern speculative symbolism treats the Perfect Ashlar as a geometric cube, historical operative "perpend ashlars" were through-stones cut flush with wall faces and were almost never cubical 3.

While the supplied sources note that Thomas Smith Webb likely derived ashlar symbolism from Preston or English brethren of Preston's period 3, they do not contain William Preston’s explicit commentary or lectures on the immovable jewels themselves.