Magnesium orthosilicate can be activated by terbium, when lithium is used as a flux, to give the characteristic green line emission of Tb+3 ion. The emission is modified without loss in 254 nm‐excited efficiency on addition of manganese, to give a supplementary orange band. The incorporation of high concentrations of activator terbium, a cation much larger than any other species present, without significant distortion of the olivine structure is probably related to known features of this lattice. The good brightness at such high activator levels suggests better coupling of the emitting rare earth to the ultraviolet absorbing host.