One of a number of marbles.
Marbles are obtained via a two step process, prospecting (using many slate and the [Rock of Ages]? skill) and then quarrying at a stone quarry. See marble for more information on how to do all that.
Oyster Shell Marble is attunable/prospectable with Perception at 1.
Prospecting is best done with 2 people and quarrying needs 4.
There are seven levels of the Pyramid Construction skill -- one available at universities of each discipline. (Each new type of university you learn the skill from will increase your level by one.) Level 7 is required to consecrate a pyramid, while lesser levels are required to assist with the various construction steps, as listed below: