Concretions are distinctive geological features that may be simply described as often (but not always) spherical deposits within fine-grained sedimentary rocks such as sandstone or mudstone. A concretion may take thousands to millions of years to form! They are formed below the sediment surface during diagenesis by localised precipitation of minerals from solution, around a nucleus - sometimes a fossil... more on this later. The nucleus is important, as this is where a chemical reaction starts to occur. Gradually certain elements in ground water solution, magnesium or iron for example, are incorporated into the mineral-binding cement, which is often carbonate, and over time there may be a change in the ratio of these elements. If ionic diffusion occurs evenly in the surrounding sediment, the concretion can form a spherical shape.
'Septarian' concretions contain an internal network of shrinkage cracks, which widen towards the interior and may be filled with crystals, commonly calcite, sometimes other minerals such as siderite barite or pyrite.
Concretions tend to stand out in rock outcrops due to their eye-catching shape(s) and erosion resistant nature - due to cementation they are often much harder than surrounding sediments. They weather out to become prominent features in the landscape... and although found worldwide, New Zealand has some particularly good examples!