Morphological, mineralogical and geochemical studies on nodules of different sizes collected from the Central Indian Ocean Basin (CIOB) show two suits of nodule formation by oxic diagenetic and hydrogenous process resulting into distinct properties. The smaller nodules are higher in grade (Cu+Ni+Co%), Mn and Zn contents and suggest their formation at shallower depths by diagenetic process and transportation later to deeper depths. Larger nodules, in contrast, are poor in grade and higher in Fe, Co and moisture contents and indicate their formation at deeper depths by a hydrogenous process. N/n (size of nodule/nucleus) ratios and nucleus size increase with increase in nodule size, supporting the view that smaller and larger nodules were formed differently at different places. Among nodules of other sizes, only medium sized ones of 3-4 and 4-5 cm are higher in grade and their exploitation seems to be economical