  Zeeman spectra of the R lines of ruby (Cr$^{3+}$: $\alpha$-Al$_{2}$O$_{3}$)
were studied in ultrahigh magnetic fields up to 230 T by
magneto-photoluminescence measurements. The observed Zeeman patterns exhibit
nonlinear behaviors above 100 T, evidencing the breakdown of the previously
reported Paschen-Back effect for ${\bf B} \perp c$ geometry. We adopted the
crystal-field multiplet theory including the cubic crystal field (${\mathcal
H}_{\rm cubic}$), the trigonal crystal field (${\mathcal H}_{\rm trig}$), the
spin-orbit interaction (${\mathcal H}_{\rm SO}$), and the Zeeman interaction
(${\mathcal H}_{\rm Z}$). It is found that the nonlinear splitting of the R
lines is owing to the hybridization between the $^{2}E$ and $^{2}T_{1}$ states,
which leads to the quantization of these Zeeman levels with the orbital angular
momentum. Our results suggest that the exquisite energy balance among
${\mathcal H}_{\rm cubic}$, ${\mathcal H}_{\rm trig}$, ${\mathcal H}_{\rm SO}$,
and ${\mathcal H}_{\rm Z}$ realized in ruby offers a unique opportunity to
observe the onset of the $crystal-field$ Paschen-Back effect toward the
high-field extreme.
