An interesting oddity from the Schwarza river in Thuringia:
The so-called "Raseneisenstein" or "Raseneisenerz" (bog ironore, a mixture of: Goethite, Hematite, Hisingerite, Jarosite, Lepidocrocite, Maghemite) from the Schwarza river is formed when iron parts of human origin or iron minerals (hematite, magnetite, red iron ore, glass head, etc.) or old iron slags (from the time before 1620), which sometimes still contain relatively large amounts of iron, lie undisturbed in the river for a long time. They are deposited in the same places as the gold grains, because they are the second heaviest elements in the river after the latter. In the process, they rust, and the iron hydroxide that forms - rust - has a larger volume than the parent material itself. This allows gold grains to grow into this rust formation. At that time, the ancients always suspected that the gold in these bog ironore stones would be formed under the influence of the sun or from germs in the air; they collected and ground such chunks especially to make the gold more accessible. During drying of bog ironore stones, the iron hydroxide transforms into normal iron oxide under water loss and crumbles in the process. Therefore, such specimens are impregnated with acrylate or with water-binding wax. Both materials are water compatible and thus the lawn iron stones are stabilized. They look like freshly taken out of the water. Unfortunately, you can only see the gold nuggets that are on the outer surface, those which may be inside remain hidden. The two solutions used for the preparation can be removed at any time if necessary. The gold nuggets do not always look shiny because they have not really moved in the water and are therefore not rubbed bare. Gold does not rust, of course, but rust can very well creep over the gold and coat it.
Such bog ironore stones can be found everywhere in the Schwarza river up to the Saale river. About the periods of time in which lawn ironstone forms on human iron parts are probably relatively short, because under the footbridge in Sitzendorf village specimens have been found on which partly the rust-protective varnish of the paint coating was still visible.