https://www.sciencedirect.com/topics/earth-and-planetary-sciences/manganese-nodule 

Deep-Sea Sediments

Reinhard Hesse, Ulrike Schacht, in Developments in Sedimentology, 2011

6.2.1 Ferromanganese nodules and crusts

Manganese-nodule fields are among the more spectacular features that have been discovered on the deep-sea floor (Fig. 9.31). They vary largely in size (from micronodules to more than meter size), shape (from spherical to ellipsoidal to tabular), surface texture (from smooth to botryoidal to rough) and abundance (Glasby, 2006). Micronodules are generally < 1 mm in diameter and form around siliceous tests, calcareous microfossils or volcaniclastic fragments. Macronodules, in contrast, are > 3–6 mm in diameter so that there is a size gap between 1 and 3 mm, which reflects the fact that the macronodules require a nucleus several mm in diameter. They precipitate around rock or bone fragments such as weathered volcanic rock, pumice, shark teeth or whale ear-bones, forming concentric laminae and layers of varying Mn and Fe concentration.