Weaubleau eggs are rocky balls, read concretions of rock found in western Missouri. See for example www.impact-structures.com (8th July 2013) (scroll down to that date)
At the link these rock eggs are compared to more recent impact sites in Spain. The process, they say, is similar to the formation of monomictic impact breccias with rounded clasts. However, the Missouri rock balls are extremely old, it is thought – 330 million years ago.
The Missouri rock balls were originally considered to be glacial in origin – even though Missouri is well south of the ice sheets. They have since been attributed to impact – but the process of formation is little understood.
Rock balls are common to all ages and all periods of the past. For instance round rocky balls of flint are often found in clay and flints geology in the UK and they form around other materials – organic and inorganic. It seems the Missouri rock balls have formed around blue shale that was blasted out of the crater and they are known otherwise as conglomerate rocks. What is interesting is that they are sometimes described as chert concretions – chert being another form of flint. It seems that silica saturated water played a role and was responsible for the silica, or chert, shell around the shale fragments. This is exactly what has happened with the rock balls in clay and flints geology in southern Britain, silica in a liquid or viscuous state has formed around other material – but nobody has suggested the London Basin is a giant crater. In fact, it is by no means certain that there is a crater in western Missouri. However, silica saturated water probably did play a role in the flints balls of the UK as the clay in the geology has an origin in water flow.
Finally, Thunderbolts has had a post on Weaubleau eggs – see www.thunderbolts.info/tpod/2009/arch09/090303weaubleau.htm
See also https://en.wikipedia.org/wiki/Weaubleau_structures, and www.roundrockjournal.com/?p=5 and www.roundrockjournal.com/?p=1112