Magnetic particles are prepared containing a magnetic core coated with a glass layer having a substantially pore-free glass surface or having pores with a diameter of less than 10 nm. The particles are used for separating biological material such as nucleic acids. A preferred process of preparing the particles is by forming a mixture of magnetic cores with a sol formed from an alcohol and a metal alkoxide, spray-drying the mixture to coat the cores with a layer of gelled sol, and heating the coated cores to obtain the magnetic glass particles. Preferably, the particles have an average particle size of less than 100 .mu.m. The magnetic core may be a composite material containing a mica core and magnetite particles immobilized on the mica core, and the glass layer may contain boron oxide. Magnetic core materials include magnetite (Fe.sub.3O.sub.4) and Fe.sub.2O.sub.3.