  Using the picture of the flux tube model, we propose that the $\Theta_5$
pentaquark as the first candidate of the three-dimensional non-planar hadron
with the extremely stable diamond structure. The up and down quarks lie at the
corners of the diamond while the anti-strange quark sits in the center. Various
un-excited color flux tubes between the five quarks bind them into a stable and
narrow color-singlet. Such a configuration allows the lowest state having the
negative parity naturally. The decay of the $\Theta_5$ pentaquark into the
nucleon and kaon requires the breakup of the non-planar diamond configuration
into two conventional planar hadrons, which involves some kind of structural
phase transition as in the condensed matter physics. Hence the width of the
$\Theta^+$ pentaquark should be narrow despite that it lies above the kaon
nucleon threshold. We suggest that future lattice QCD calculation adopt
non-planar interpolating currents to explore the underlying structure of the
$\Theta_5$ pentaquark.
