The photoluminescence of several natural and synthetic diamonds of type I and type II has been examined in the spectral range 1.351.85 eV (920670 nm) at 300 and 79 K at ambient pressure. Two emission bands with zero-phonon lines at 1.575 and 1.770 eV are observed in all yellow type-Ia natural diamonds. These bands are attributed to aggregate nitrogen defects and coexist with other known defect emission bands N2 and N3. The implications for high-pressure research using diamond anvils are discussed. © 1989 The American Physical Society.