Crystallization of ruby requires excess Al and appreciable amounts of Cr in the system. A ruby–bearing feldspathic dike crosscuts dunite in the Ray–Iz massif, the Polar Urals, and the dominant mineral of the dike changes from plagioclase at the center to amphibole outward. Ruby has been observed in between, and the zone is composed of plagioclase and phlogopite with minor chromian spinel and ruby as primary phases, and paragonite as a secondary phase. The Cr₂O₃ content of the ruby is <7.5 wt% and close to the values of those found in serpentinite and chromitite from other localities. The petrographical and highly LREE–enriched and HREE–depleted features of the ruby–bearing rock imply a metasomatic origin for the dike through the interaction between feldspathic component–rich aqueous fluid and wall rock dunite with chromitite. Based on the primary occurrence of plagioclase, it is inferred that the fluid infiltration possibly occurred at 1.0–1.5 GPa, and the fluid interacts with peridotite. The lithological change of the dike indicates effective consumption of Si, Ca, and K and assimilation of Cr and Mg in the fluid at the contact with the wall–rock dunite, and the fluid composition could have evolved to be peraluminous through the interaction. Chromium is effectively transported by aqueous fluid with some anions, e.g., Cl⁻, CO₃²⁻, and SO₃²⁻, and the interaction of peridotite as a source of Cr with such fluids is one of the important formation processes of ruby within the mantle wedge where fluids are available from the downgoing slab.