Ruby deposits in marbles are the most important source of ruby. For a long period, some problems concerning the prospecting of this type of deposit have been misunderstood. On the basis of a study and elaboration of data, some important conclusions are presented: (1) ruby deposits in marbles occur in the plutonic metamorphic zones of orogens where deep fault structure is developed; (2) ruby-bearing rocks are calcic crystalline marbles, rather than magnesian marbles; (3) amphiboles in ruby-bearing rocks are not tremolites, but silica-poor, alumina-rich and chrome-bearing calcic amphiboles, such as chrome-bearing magnesio-sadanagaites; and (4) the deposits are genetically of thermodynamical metamorphism type, rather than of pneumatolito-hydrothermal type or skarn type.