<P> Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core . The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization . When visible to the naked eye, it can be a small test (shell) of a microfossil (radiolarian or foraminifer), a phosphatized shark tooth, basalt debris or even fragments of earlier nodules . As nodules can be found in vast quantities, and contain valuable metals, deposits were identified as having economic interest in the 1960s by John Mero . </P> <P> Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 centimetres (8 in) across . However, most nodules are between 3 and 10 cm (1 and 4 in) in diameter, about the size of hens eggs or potatoes . Their surface textures vary from smooth to rough . They frequently have botryoidal (mammilated or knobby) texture and vary from spherical in shape to typically oblate, sometimes prolate, or are otherwise irregular . The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth . </P>

Manganese nodules are examples of what type of ocean sediment
find me the text answering this question