The marble staircase is located in the First Academy Building on the campus of the Devon School. Gene describes it as having a red brick exterior, swinging doors, and a marble foyer preceding the staircase. This is one of the buildings he visits when he returns to the school 15 years after his graduation. Upon examination of the stairs after so many years, Gene says the following:
". . . the worn moons in the middle of each step were not very deep. The marble must be unusually hard. That seemed very likely, only too likely, although with all my thought about these stairs this exceptional hardness had not occurred to me. It was surprising that I had overlooked that, that crucial fact" (11).
Notice what the older Gene focuses on while describing these stairs years after his friend Phineas re-breaks a leg on them. He analyzes the unique hardness of the marble as almost an uncanny thought to him, now. And the "crucial fact," referring to marble being a hard surface, solidifies in his memory the terrifying and fatal moment when Finny broke his leg again on those stairs and died later because of it.
At the time of Finny's fall on those marble stairs, though, Gene describes the acoustics of the marble foyer as "admirable" (173). But when Gene describes what it sounded like to hear Finny fall on the marble stairs, it is as significant as Finny's life:
"The excellent exterior acoustics recorded his rushing steps and the quick rapping of his cane along the corridor and on the first steps of the marble stairway. Then these separate sounds collided into the tumult of his body falling clumsily down the white marble stairs" (177).
Thus, the staircase signifies the end of Phineas because that is where his final accident happens which causes him to die from surgical complications later.