Histograms of average and instantaneous growth rate distributions for calcium sulphate crystals (gypsum) formed on a heat-exchange surface are considered. It was found that at small supersaturations the distribution curves of the crystal growth rates have positive skewness, with increase in supersaturation the symmetry of the curves grows (moment coefficient of skewness tends to zero), and the distribution law becomes normal (Gaussian) or close to it. Comparison with the published data showed that the regularity obtained was common for all salts and was caused by the effect of impurities on crystal growth.