We present the first 850 $mu$m polarization observations in the most active star-forming site of the Rosette Molecular Cloud (RMC, $dsim$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.
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