This book examines picroilmenites and their ferromagnetic behavior in the kimberlites from the Yakut diamondiferous province. Picroilmenites are minerals used to identify the location of diamonds. The author shows a solid interpretation of the magnetic-mineralogical analysis of ferromagnetic minerals based on a large number of experimental data and modeling of the magnetic state. He also presents the problems of the variability of the composition of picroilmenites from various kimberlite pipes. Furthermore, this book proposes a method to estimate the distribution of the decay structures dimensions, according to the thermomagnetic analysis and coercive spectra of titanomagnetites with the magnetite-ulvospinel decomposition structures. This book will be useful for students and researchers working in the field of rock magnetism, as well as geologists and geophysicists.