The zero-phonon lines GR2, GR3,..., GR8 which are observed in electron-irradiated diamond between 2.8 and 3.1 eV have been studied using optical absorption and uniaxial stress. The relative intensities of these lines correlate with each other and with the GR1 zero-phonon lines at 1.665 and 1.673 eV in all the crystals studied. Changes in intensity with temperature also suggest a link between GR1 and GR2 $\rightarrow $ 8. Uniaxial stress measurements indicate that GR2 (2.881 eV) and GR3 (2.888 eV) are E to T transitions at the GR1 defect. It is concluded that all the transitions GR1 $\rightarrow $ GR8 occur at the vacancy.