Phosphor particles, methods for their use to produce fluorescent lamps, and fluorescent lamps that make use of such particles. Such a phosphor particle has a core surrounded by a shell, and the shell contains GdMgB5O10 doped (activated) with at least terbium ions as a rare earth-containing phosphor composition that absorbs ultraviolet photons to emit green-spectrum light. The core is formed of a mineral material that is chemically compatible with the rare earth-containing phosphor composition of the shell, but does not contain intentional additions of terbium.