The crystalline phase precipitation in the glass crystallization of M-type Ba-hexaferrite fine particles is investigated by means of Faraday microscopy. The experimental method using a magneto-optical garnet sensor film which converts the magnetic stray field distribution at the sample surface into a luminance distribution in the recorded image is described. By combination with X-ray diffractometry, magnetization measurements and scanning electron microscopy including EDX-microprobe analysis the phase constitution of glass ceramics prepared in different ways is determined. It is shown that the precipitation of fine crystalline particles of different phases is influenced by inhomogeneous internal stress in the glass matrix which is either related to the crystallization process in a self-generating manner or may be induced by mechanical crushing of the starting glass phase.