Gene is visiting Devon School after being away for fifteen years. He is remembering events that happened while he was a student, and is looking at places that were important during his time as a student with the benefit of hindsight. Factors that didn't seem significant at the time assume greater importance as he is looking back and understanding their impact on the events he is recalling.
As he enters the First Academy Building, he views the "long white marble flight of stairs." These stairs, which he had taken for granted during most of his time as a student, were the stairs where Finny slipped and rebroke his leg, the event that directly led to his death.
Gene notices that, despite the age of the building and the stairs, the stairs were not worn away as the result of years of use. "The marble must be unusually hard...It was surprising that I had overlooked that, that crucial fact." The hardness of the marble contributed to the severity of Finny's injury. That was the surprising insight that Gene experiences.