Abstract: Jet is a kind of natural organic gemstone, which is composed of organic matter and a small amount of minerals. At present, there is a lack of systematic research on the material compositions of jet in the academic community. This study used infrared absorption spectroscopy, density and ash content determination to analyze the material compositions of jet samples from Fushun, China and coal samples, laying the foundation for the identification and quality grading of jet from Fushun, China. According to the infrared spectral characteristics of the jet samples from Fushun, China and the coal samples, it is found that the material composition of the jet samples contain a considerable amount of ester compounds, while almost no such compounds are found in the coal samples. These compounds are mainly from sapropelic coal during the formation of jet. Both jet and coal samples contain aliphatic hydrocarbons, but the content of aliphatic hydrocarbons in jet is higher than that in coal. Jet is humus-sapropelic coal which is formed during the coalification process when humus coal (mainly composed of aromatic groups) was filled by sapropelic coal (mainly composed of chain aliphatic groups). The density and ash content of jet, coal or coal gangue are closely related to the amount of minerals they contained. The more minerals they contain, the higher the ash content and density will be. The material composition of jet determines its external characteristics.