Temperature gradients can trap micrometer-sized particles into two-dimensional crystals. We form colloidal crystals from otherwise repellent 2 μm polystyrene beads in diverse thermal convection settings. Our experiments indicate that the accumulation is driven by particle thermophoresis. Particles move along the temperature gradient and are pushed out of the warm liquid to a cold wall. We find reduced accumulation for decreased surface temperature gradients and enhanced salt concentrations. Moreover, thermophoretic fluid dynamics calculations predict flat accumulation profiles with 107-fold enhanced concentrations that are consistent with our experiments. The accumulated crystals could be used as molecular sieves for microfluidic biotechnological applications. A natural environment for similar accumulations are pores of rock near hydrothermal vents.