  Using first-principle numerical simulations, we investigate the Casimir
effect in zero-temperature SU(3) lattice gauge theory in 3+1 spacetime
dimensions. The Casimir interaction between perfect chromometallic mirrors
reveals the presence of a new gluonic state with the mass $m_{\mathrm{gt}} =
1.0(1)\sqrt{\sigma} = 0.49(5)\,\mathrm{GeV} = 0.29(3) M_{0^{++}}$ which is
substantially lighter than the $0^{++}$ groundstate glueball. We call this
excitation ``glueton'' interpreting it as a non-perturbative colorless state of
gluons bound to their negatively colored images in the chromometallic mirror.
The glueton is a gluonic counterpart of a surface electron-hole exciton in
semiconductors. We also show that a heavy quark is attracted to the neutral
chromometallic mirror, thus supporting the existence of a ``quarkiton'' (a
``quark exciton'') colorless state in QCD, which is formed by a single quark
with its anti-quark image in the chromometallic mirror. Analogies with edge
modes in topological insulators and boundary states of fractional vortices in
multi-component condensates are highlighted.
