Ultramarine blue was synthesized by using reservoir silts as a major raw material to replace traditional kaolin clay. The reservoir silts were pretreated to collect the fines, in which the main mineral composition is illite clay. The fine part was mixed with sodium carbonate, sulfur, and activated carbon, and calcined at 800 • C for 8 h. The products were examined by X-ray powder diffraction (XRD) and a scanning electron microscope (SEM) to identify the mineral phases and particle morphology. The color characteristics were measured according to the CIELab system. Brilliant blue ultramarine powders were successfully synthesized with a chroma of 52.4, which is competitive with the commercial ultramarine.