Gem-quality corundum (sapphire) occurs in scapolite-rich calc-silicate rock hosted in marble of the Lake Harbour Group near Kimmirut, southern Baffin Island. A deposit of blue and colorless gem corundum (Beluga occurrence) is compared to a similar calc-silicate pod generally lacking corundum but containing nepheline (Bowhead occurrence) and located 170 m to the SSW. Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite (Me50–Me67) as a result of CO2-, H2O-, Cl-, F-bearing fluid influx at 1782.5 ± 3.7 Ma (P-T < 720 °C, 6.2 kbar); and (3) retrograde breakdown of scapolite + nepheline (with CO2- and H2O-bearing fluid) to form albite, muscovite, corundum, and calcite. Late, low-temperature zeolite mineralization is common in corundum-bearing zones. Based on thermodynamic models, the corundum-forming reaction only occurs in a <100 °C window with an upper limit determined by scapolite-nepheline stability, and a lower limit determined by the formation of Al-silicate rather than corundum. The protolith is inferred to be dolomitic argillaceous marl with no evidence to suggest the initial presence of evaporites. The enrichment of trace metals V and Cr, and the depletion of Co, Ni, and Mn, suggest reducing diagenetic conditions in the initial sediment. Beluga calc-silicate rock is strongly depleted in REE (Total REE ∼ 18 ppm). Oxy-dravite δ11B (+3.9 ± 0.7‰) is consistent with a marine boron source. The oxygen isotope composition of corundum (δ18OVSMOW = 16.4 ± 0.1‰) is comparable to that of corundum in marble or desilicated pegmatite associated with marble. Phlogopite and muscovite 40Ar/39Ar ages and calculated closure temperatures (considered estimates) are ca. 1640 Ma (Tc = 455 to 515 °C) and 1510 Ma (Tc = 410 to 425 °C), respectively. In the Lake Harbour Group, the most prospective areas for gem corundum exploration are expected to be contiguous to the thrust fault separating the Lake Harbour Group and Narsajuaq terranes, where the retrograde, amphibolite facies overprint of the granulite peak assemblages was most pervasive.