Purpose is to identify changes in the basic indices of vitrinite reflection as well as content of volatile matter in the process of seam underground transformation influenced by high temperature and pressure and define nature of the changes in the classification indices and their correspondence to grades, groups, and subgroups of similar metamorphic coal transformations to determine hazardous characteristics of seams while mining. Methods. Selection of the most adequate basic index, evaluating metamorphism degree of coal seam and forecasting of their hazardous characteristic demonstration while mining, is based upon the average data of coal composition in Donbas depending upon the stages of seam metamorphism. Findings.Analysis of indices of the current industrial classification and normative base has shown the following. They cannot characterize directly the coal metamorphism connected with changes in composition and properties of the original organic substance. It has been mentioned that volatile matter do not demonstrate specifically ultimate composition of the organic matter and phase coal composition while fluid extraction from seams during their metamorphic transformations. Only the total release of gaseous products without identification of the released gases is defined by its value. Average vitrinite reflection has been defined experimentally along with other indices to identify the coal grades. For that reason, values of the index not always correspond to coal metamorphism degree in their grade ranking. Originality. Nature of changes in the classification indices of industrial coal ranking has been defined to identify hazardous characteristics of coal seams while mining. Practical implications. Possibilities to improve a normative base for safe mining have been defined while determining the classification indices characterizing directly the changes in ultimate composition during metamorphic processes.