Magnetorheological (MR) finishing has been adopted as a newly ultraprecision finishing method for microparticles, an optical system, and an aspherical lens, mainly by MR fluids consisting of magnetic carbonyliron (CI) particles and a carrier fluid such as silicone oil or diwater. However, CI particles are related to corrosion problems because of their oxidation, leading to unpredictable polishing results and a short lifetime of the MR fluids. To resolve this problem, we coated CI particles with a biopolymer of xanthan gum (XG) for better MR polishing and measured their MR properties using a rotational rheometer under applied magnetic fields of different strengths. Experiments were carried out to examine the material removal depth and the surface roughness for BK7 (borosilicate) glass by changing the experimental parameters. While the material -removal depth obtained using raw CI particles was deeper than that for the XG-coated CI particles, the surface roughness obtained using the XG-coated CI particles after MR polishing was lower than that for pure CI particles. These results confirm that XG-coated CI particles can be applied for the MR polishing of a BK7 glass well.