The study of weathering is important in terms of predicting rock behavior against weathering factors and its suitability for borrow material. The first step to surveying weathering is, to provide a appropriate method for classification. In this study, the changes caused by weathering calcareous slate in laboratory condition(salt crystallization, freezing-melting, wetting-drying, cooling-heating(, and also atmospheric conditions (for one year) were analyzed (about 1000 samples). These traces are mostly exfoliate,decoloration of pyrite-magnetite, the effects of iron rust, and edges corrosion. The intensity of these changes in the weathering test has resulted in more tolerance than other tests. In addition to the effect on the engineering behavior of stones, pyrite oxidation also has an adverse effect on the appearance of stone materials. Hence, pyrite-magnetite in calcareous slate, particularly in terms of shape and appearance changes caused by weathering were evaluated and classified. These crystals are categorized mainly into three gravel-cubic, mass and dendritic types, and classified according to the weathering grade to P1 to P5 (without weathering to fully weathered). Finally, we have reviewed geotechnical problems caused by weathering of pyrites.