Seneca isn't simply saying that hardship builds character—he's drawing an equivalence between two physical processes, suggesting that personal growth requires *the same kind of relentless pressure* that transforms raw stone into something luminous. The image of friction working on a gem matters because it strips away sentimentality; you can't negotiate with sandpaper or ask it to be gentler. A parent watching their child struggle through a difficult school subject, resisting the urge to rescue them, understands this instinctively—the friction that makes them want to intervene is precisely what their child needs.