The ratio of red to purple garnets found in tills in the eastern Great Lakes area has been very useful in stratigraphic and provenance studies of the tills. In an attempt to determine the basis of the color difference, and to supplement the colour differences by more objective criteria in till studies, X-ray diffraction and fluorescence, atomic absorption, and microprobe analyses were made. X-ray diffraction analyses show a consistent difference in the cell parameter of 0.05 Å (0.005 nm) or more between the two groups. Microprobe analyses of major elements indicate that the calcium and calculated ferric iron contents are higher and magnesium is lower in dark red garnets than in purple garnets. Bulk analyses of minor and trace elements by X-ray fluorescence and atomic absorption of different colour groups show distinct differences in rare-earth element patterns and manganese, which is eight times more abundant in the dark red garnets.