Some of our collectors can spot the difference immediately. Can you? Here is what to look for.

First, concretions. These are aggregates in sediments of inorganic matter in many but usually spherical or discoidal shapes. Often there is a nucleus of some sort – a bit of pyrite. A tiny fossil remnant, a shark’s tooth – but it may be absent or microscopic.

The origin of concretions is controversial; see what you think after you’ve read the rest of this story.

They are commonly composed of one material but others may be present as impurities. These are some likely contenders; calcite (the most common), silica, hematite, siderite and other iron compounds; gypsum, barite, aragonite (cousin to calcite), manganese oxide, calcium phosphate, fluorite and bauxite (the ore of aluminum).

Calcite is the chief material in claystone and calcareous concretions so common in shale and sandstones.

Siliceous concretions usually are nearly pure silica, as in chert and flint. Commonly scattered throughout chalk beds these often contain the tiny remains of radiolarians (siliceous foram inifera) and sponge spicules (skeletons).

Siderite concretions (iron carbonate) may be so abundant as to form (low-grade) iron ores. Pyrite and marcasite concretions (iron sulfides) are widespread in shale, limestone and especially dark marine shales. Barite "roses" are usually products of desert sands. Calcium phosphate nodules are valuable sources of that mineral.

Other concretionary shapes are due to other processes – oolites – like fish-eggs – is the common form for bauxite, the ore of aluminum. Then there are larger pea-sized forms – pisolites, rolled about in shallow warm seas.

Sizes? How about log-shaped forms to 10 m? These usually form in sandstone.

Origins? What causes minerals to migrate to a center and become harder i.e. a strange foreign shape, in a sedimentary bed? Some are revealed only when they weather out of an outcrop. How come the fine bedding planes of the host rock continue right through the concretion?

Septarian nodules (from Latin septum = partition) are marked by a network of cracks usually filled with calcite.

The prizes for collectors of concretions are the hollow types called geodes. These globular types vary from centimetres to about 30 centimetres and usually have a rough surface of dense chalcedony. They are typical of some limestone beds but seldom in shales.

What intrigues collectors is the possibility of a geode to contain spectacular crystal growths. But because geodes, once released from their enclosing beds tend to look like ordinary boulders they must be broken open to reveal any treasure. And guessing which "boulder" to smash open – and possibly destroying the interior – is a problem facing the finder. Estimating weight may be of some use. Random sawing is time consuming.

It’s difficult to imagine the lowly concretion being involved in deep intrigue – literally. In the dark days of the Cold War, the Hughes Company was commissioned to design, build and operate an immense floating "mining machine" to allegedly recover from the deep Pacific sea-floor the millions of manganese nodules known to form there. They did.

Only years later did the public learn that this was in fact a CIA secret operation to recover a sunken Russian submarine. Nearly at the surface, the apparatus failed and the hulk sank again among the nodules.

The cause of geodes and concretions and various modular objects make interesting but lengthy and erudite reading for those interested.

But this isn’t necessary to enjoy the wonder and often the beauty of these strange objects.