Crystal forms of RbCl and RbBr with B1 and B2 structures under high pressure in aqueous solutions were observed. In B2, facets on spherical RbCl enlarged and RbBr retained its curved shape with increasing pressure. In B1, the {100} planes shrank where RbCl had round corners and RbBr had faceted ones with increasing pressure. The {100} shrinkages of RbCl and RbBr with B1 structure, and the facet expansion of RbCl with B2 structure can be thermodynamically elucidated from the pressure dependences of the lattice interaction energies estimated by the pressure dependences of both the solubilities, which had opposite tendencies to each other. To interpret new facet formation on the corners of RbBr in B1 and the curved surface of RbCl in B2, we proposed a geometric parameter to evaluate the degree of surface hydration which is responsible for the pressure dependence and the anisotropy of the lattice interaction energy. The conditions for the new facet formation were seen through the pressure dependences and the anisotropy of the parameters, calculated from the compressivities of the crystals and a water molecule, and the ratio of the area that can be hydrated to a surface ion's naked area.