Pyramidal hillocks are often formed during wet chemical etching of silicon for micromechanics applications and prevent the production of a smooth surface, The observations of silicon pyramids led to the hypothesis that a semipermeable particle locally retards the etching, causing the pyramid formation (J. Appl. Phys. 89 (2001) 4113). The formation of pyramids is not unique to silicon and therefore the causes of the formation and stability of the hillocks cannot be sought in peculiarities of silicon. We investigated {1 0 0} NaCl etched in formamide as a model system to improve our understanding of this phenomenon. Our in-situ and ex-situ observations of pyramid growth, shape and density support the hypothesis of the semipermeable particle. We have strong evidence that a masking particle of dust at the surface is responsible for the appearance of the pyramids on the crystal surface during etching. The size of the particle is < 300 nm. (C) 2002 Elsevier Science B.V. All rights reserved.