  Deviations from Hawking's thermal black hole spectrum, observable for
macroscopic black holes, are derived from a model of a quantum horizon in loop
quantum gravity. These arise from additional area eigenstates present in
quantum surfaces excluded by the classical isolated horizon boundary
conditions. The complete spectrum of area unexpectedly exhibits evenly spaced
symmetry. This leads to an enhancement of some spectral lines on top of the
thermal spectrum. This can imprint characteristic features into the spectra of
black hole systems. It most notably gives the signature of quantum gravity
observability in radiation from primordial black holes, and makes it possible
to test loop quantum gravity with black holes well above Planck scale.
