An accumulation of
mineral matter formed around a center, or
axis, of deposition after a sedimentary deposit has been
laid down. Cementation consolidates the deposit
as a whole, but the concretion is a body within host
that represents local concentration of cementing material: enclosing rock is less firmly
cemented than the concretion. Commonly spheroidal or disk-shaped and composed of such
cementing agents as calcite,
iron oxide, or silica.
Source: Leet, L. Don. 1982.
Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall