The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet sun. These bright grains are seen with the IRIS 1330 AA, 1400 AA and 2796 AA slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1-m Solar Telescope (SST) which allows us to have chromospheric (Ca II 8542 AA, Ca II H 3968 AA, Halpha, and Mg II k 2796 AA), and transition region (C II 1334 AA, Si IV 1402) spectral imaging, and single-wavelength Stokes maps in Fe I 6302 AA at high spatial (0.33), temporal and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 AA slit-jaw filter comes from both the Mg II k core and wings. The signal in the C II and Si IV lines is too weak to explain the presence of grains in the 1300 and 1400 AA slit-jaw images and we conclude that the grain signal in these passbands comes mostly from the continuum. Even though weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C II spectra. For some grains, spectral signature can be found in IRIS Si IV. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region.
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