The conditions of fluid expulsion during progressive cooling of granitic crystal mushesremain poorly understood. I propose to reconstruct the chemical and physical conditions of fluid percolation in solidifying granitoids using the mineral tourmaline. An ideal setting to study tourmaline in granitoids is the Stone Mountain Granite (Georgia, USA). Stone Mountain (SM) is a quartz monzonite that forms a dome monadnock. SM displays “cat paws” or clusters of tourmalines surrounded by a leucocratic halo and are dispersed throughout the plutonic body. New results based on the combination of field observations and electron probe micro-analyzer (EPMA) of tourmalines in a granitoid help to unravel the physical and chemical conditions of fluid transport in crystal mushes. This study sheds light on the ultimate processes of crystallization of the Stone Mountain Granite as a “fossil magma chamber” and mobilization of residual fluids in the Earth’s upper crust.