The largest particles in a ground coffee bed — informal name for the chunks that didn't get fully broken down by the burrs. The opposite of fines. Visible to the naked eye in most home grinds: little gravelly bits sitting alongside the medium particles.
Boulders extract slowly. While the bed average is hitting target extraction, boulders are still under-extracted. Worse, they create local channels where water moves around them rather than through, which means a percentage of your dose effectively contributes nothing to the cup. The taste signature: a coffee that reads thin or under-extracted in spite of a perfectly fine recipe and grind setting.
What causes them
- Worn or misaligned burrs. Old burrs lose their cutting edges and start tearing rather than slicing, leaving more uncut chunks.
- Hand grinder geometry. Some hand grinders pass beans through quickly without fully reducing them, especially at coarser settings.
- Static and clumping at fine settings can also produce apparent "boulders" — clumps of fines stuck together that behave like a single large particle.
What to do
For most home setups, the answer is to upgrade the grinder before chasing recipe changes. If you sieve a tablespoon of grounds through a 1 mm and 0.5 mm kitchen sieve and find a chunky boulder layer, that's the bottleneck. A grinder upgrade tightens the distribution faster than any pour-pattern adjustment ever will.