The ferromagnetic phases which precipitate from various iron‐containing silicate glasses have been identified and characterized by studying the integrated intensity of the ferromagnetic resonance absorption as a function of sample temperature between 4.2 and 570°K. The analysis makes use of the effect of microwave skin depth on the FMR intensity to estimate the average particle size for fine disperisions of metallic iron, and experimental results on well‐characterized glasses are in good qualitative agreement with theory. Furthermore, by using the results of modelling experiments carried out on iron and magnetitie precipitates produced independently in different glasses, it is possible to detect and discriminate between metallic iron and ferric iron spinel, even when both are co‐present in an unknown sample. This result is important because spectra characterized by positive magnetocrystalline anisotropy constants have been observed in samples containing fine†grained magnetite.