Last Friday’s  mystery object was this:

This roughly spherical object represents my most frequently occurring enquiry at the Horniman Museum – people find these all the time and they are always convinced that they are something extraordinary. It is of course a concretion. These structures are formed in sedimentary rocks when minerals grow around a nucleation object. Often that object will be a fossil, so sometimes there will be something interesting inside. However, mostly whatever started the mineralisation process did so as it decayed and most often there won’t be much left of it,  as you can see here:

The correct answer came in stages, with KateKatV suggesting a geode (which is close but they are formed differently – most notably they do not form around a solid central nucleation point, so they are hollow), then SmallCasserole suggested it was flint nodule (again close, but different in that nodules are replacement bodies rather than structures formed around a nucleation point), finally Debi Linton nailed it. However, much kudos goes to Dave Godfrey who misidentified it as a nodule, but correctly predicted what such objects are  invariably presented to me as (on average about once a month) – dinosaur eggs. Dave was also correct when he said:

And some of them won’t take no for an answer.

How true. I have met people who had planned their retirement around the money they had expected to make from selling similar finds. A vivid imagination and a lot of hope might be an endearing character trait, but it won’t turn one of these into cold hard cash – unless you’re able to sell it to someone more gullible on Ebay…