In the present paper, the results of investigation of the heterogeneous nature of coals are given. Microscopic observations of coals of different rank and mineral matter content were performed. The content of minerals and macerals and the size of their grains were determined in the coal samples. Two samples of lignite rich in minerals were analyzed in detail by measuring the ash content for each particle and batches containing numerous [100] coal particles. The additional analyses were performed with the particles before and after sink-float separation. The obtained results showed an increase of heterogeneity with mineral matter content. The statistical analysis showed that many particles should be used for an experiment for obtaining reliable data with a desired level of certainty. In the case of batch experiments with 5 g Kolubara B coal (about 100 particles, size 4.76-7.0 mm), the experiment should be repeated at least 7, 19, and 100 times in order to obtain a mean value in the confidence intervals 5.0%, 2.5%, and 1.0%, respectively, for a 95% probability (if sample heterogeneity is the only cause for variations in results). However, regardless of increase of reproducibility and repeatability of obtained results, a large amount of samples and numerous repetitions of experiments cannot exclude differences in experimental results between homogenous and heterogeneous samples.