  X-ray circular dichroism, arising from the contrast in X-ray absorption
between opposite photon helicities, serves as a spectroscopic tool to measure
the magnetization of ferromagnetic materials and identify the handedness of
chiral crystals. Antiferromagnets with crystallographic chirality typically
lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet
exhibit weak X-ray natural circular dichroism. Here, we report the observation
of giant natural circular dichroism in the Ni $L_3$-edge X-ray absorption of
Ni$_3$TeO$_6$, a polar and chiral antiferromagnet with effective time-reversal
symmetry. To unravel this intriguing phenomenon, we propose a phenomenological
model that classifies the movement of photons in a chiral crystal within the
same symmetry class as that of a magnetic field. The coupling of X-ray
polarization with the induced magnetization yields giant X-ray natural circular
dichroism, revealing the altermagnetism of Ni$_3$TeO$_6$. Our findings provide
evidence for the interplay between magnetism and crystal chirality in natural
optical activity. Additionally, we establish the first example of a new class
of magnetic materials exhibiting circular dichroism with time-reversal
symmetry.
