I found this completely hollow rock in a ploughed field on the Blackdown Hills (  The geology of the Blackdown Hills together with the adjoining East Devon AONB is unique in south-west England, forming part of the only extensive outcrop of Upper Greensand in the region.

The Blackdown Hills form a flat plateau dominated by hard chert bands, made up of clay with flints, of Upper Greensand with some remnants of chalk. The cretaceous rocks rest over eroded Jurassic and Triassic beds, with an outcrop of Rhaetian beds. In the western areas the Upper Greensand is devoid of calcareous material but the sands yield fossils of marine bivalves and gastropods (snails) preserved in silica. )

I assumed it was from an early iron working as it is very heavy. We ran a sophisticated metal detector over it but it is completey non metallic. Now I can only assume its chert but would that be hollow ?

Not actually cave science I know but hoping Andy F. will pick up on it.

P7250003 by Nicholas Chipchase, on Flickr

P7250004 by Nicholas Chipchase, on Flickr

Looks like a mould of something, such as modern wood or ancient sponge etc
The mineral could Siderite (iron carbonate, non metallic). I have seen similar items in haematite from the Red Crag shelly sandstones, formed by groundwater in recent times

You get iron pyrite nodules in the Chalk, so it is probably a very weathered and rusted one of those. Like flints, they sometimes form around burrows in the chalk ooze, so sometimes have a tubular shape.

Might it be goethtite formed from the decomposition of marcasite? Some of the mines near my house produce pseudomorphs of marcasite.