In this study, nephrite samples from Alamas and Karakash Rivers were collected. Also, serpentinite-related green nephrite samples were selected for comparison in colors. Methods such as electronic microprobe analysis and back-scattered electron images were employed to study the genesis and color-forming factors of nephrite. Based on whole-rock chemical analyses of nephrite obtained, the authors studied petrographic characteristics, mineral composition and color-forming factors. In this study, besides tremolite, associated minerals such as diopside, chlorite, actinolite, spinal, zircon, apatite, garnet, barite, pyrrhotine, graphite, calcite, and Fe(OH)₃ were found in nephrite. All samples have low concentrations of Cr, Ni (<100×10^-6) and spots of Mg/(Mg+Fe²⁺)-Si, suggesting all these samples are dolomite-related nephrite. According to the whole-rock analysis of nearly all data obtained, the data TFe₂O₃=0.33%~1.42%,TFe₂O₃=0.43%~0.96% and TFe₂O₃=0.77%~3.97% were found in white, white-green and green nephrite, respectively. Also, values of Cr and Ni in these samples have low concentrations below 60×10^-6 and 20×10^-6 respectively. These chemical compositions suggest that the increasing content of TFe₂O₃ in crystal structures in tremolite leads to change of the colors in nephrite from white to green. Large amounts of graphite and veinlets of Fe(OH)₃ lead to the black color of nephrite. Although both serpentinite-related and dolomite-related nephrite samples have predominated tremolite, serpentinite-related nephrite might have emerald color. In order to further study the coloring factors of Hetian nephrite, the authors collected samples of serpentinite-related nephrite in Canada for comparison. With serpentine-related nephrite in Candia as an example, in addition to the amount of Fe, a large amount of Cr (1 400×10^-6~5 100×10^-6) was found in them. The emerald color of serpentinite-related nephrite is related to 0.01~0.03 a.p.f.u. of Cr in C site in tremolite. By contrast, there exists nearly no Cr in C site in tremolite in dolomite-related nephrite. Based on the geological field observation and petrographic characteristics, the authors hold that the formation of Mg-skarn has experienced metamorphism, metasomatism and sulfur stages and tremolite formed through the replacement at the expense of the dolomite marble, diopside and other minerals. Multiple replacement of coarse-grained tremolite by fine-grained tremolite leads to the formation of nephrite.