  We present the first 850 $\mu$m polarization observations in the most active
star-forming site of the Rosette Molecular Cloud (RMC, $d\sim$1.6 kpc) in the
wall of the Rosette Nebula, imaged with the SCUBA-2/POL-2 instruments of the
JCMT, as part of the B-Fields In Star-Forming Region Observations 2 (BISTRO-2)
survey. From the POL-2 data we find that the polarization fraction decreases
with the 850 $\mu$m continuum intensity with $\alpha$ = 0.49 $\pm$ 0.08 in the
$p \propto I^{\rm -\alpha}$ relation, which suggests that some fraction of the
dust grains remain aligned at high densities. The north of our 850 $\mu$m image
reveals a "gemstone ring" morphology, which is a $\sim$1 pc-diameter ring-like
structure with extended emission in the "head" to the south-west. We
hypothesize that it might have been blown by feedback in its interior, while
the B-field is parallel to its circumference in most places. In the south of
our SCUBA-2 field the clumps are apparently connected with filaments which
follow Infrared Dark Clouds (IRDCs). Here, the POL-2 magnetic field
orientations appear bimodal with respect to the large-scale Planck field. The
mass of our effective mapped area is $\sim$ 174 $M_\odot$ that we calculate
from 850 $\mu$m flux densities. We compare our results with masses from
large-scale emission-subtracted Herschel 250 $\mu$m data, and find agreement
within 30%. We estimate the POS B-field strength in one typical subregion using
the Davis-Chandrasekhar-Fermi (DCF) technique and find 80 $\pm$ 30 $\mu$G
toward a clump and its outskirts. The estimated mass-to-flux ratio of $\lambda$
= 2.3 $\pm$ 1.0 suggests that the B-field is not sufficiently strong to prevent
gravitational collapse in this subregion.
