Boulders

Boulders are the largest particles in a grind: chunks much coarser than your target size. Every grinder produces a spread of particle sizes rather than one uniform size, and boulders sit at the big end of that spread. They are the opposite of fines, the dust at the small end.

Why they matter: a coarse particle has a small surface-area-to-volume ratio, so water struggles to penetrate its core. The outside gives up flavor while the inside stays largely untouched. The water that flows through these under-worked chunks comes out weak and tea-like, effectively diluting your brew with under-extracted liquid even when the rest of the bed is fine.

Boulders are worst with blade grinders, which smash beans randomly, and with worn or poorly aligned burrs. Quality conical or flat burrs cut down on both boulders and fines, giving a tighter, more even particle distribution. If your coffee tastes thin and sour despite a seemingly fine grind, an inconsistent grinder throwing boulders is a common culprit. See the grind-size-guide for matching grind to method.