Now, if you are a geologist, concretions are mysterious in their own right because they are often formed in bizarre lumpy or spherical shapes in a wide variety of sizes by completely natural methods, but the exact circumstances surrounding their formation are still relatively unknown. They occur when very small mineral particles glue together to form a type of cement between larger grains of sand or dirt. Concretions are often harder than the surrounding rock they form in, so over thousands and millions of years they will be the only part of the rock formation that remains and end up in quite striking patterns and shapes.
There are many examples of spherical concretions like the one found in Bosnia from around the world. The Moeraki Boulders of New Zealand are an example of “cannonball concretions”. These famous, very spherical boulders are found on a beach in Otago. They are part of a Paleocene-aged rock formation, so they date back 60 million years and likely took millions of years to grow to their most massive sizes of up to 7 feet wide. Other examples of almost impossibly round cannonball concretions occur in
Theodore Roosevelt National Park in North Dakota, and these ones are also quite reddish and iron-stained due to their mineral composition.