“Many cherts and flints will patinate, developing a weathered surface as water and sometimes chemical stains work their way into the flint and as silica and other materials are leached out, producing a thin patina or rind of a different color" Whittaker 1994. 70
In southern Britain the patination is usually a blueish colour.​


If the patinated nodule is flaked then the original black surface of the flint is revealed.