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I found this slightly rude-shaped rock/stone on the South Downs in East Sussex, UK years ago. It was embedded in a chalky path and came out in the two pieces in the photo. It is heavy for its size (975g) and doesn't scratch with a metal blade (I don't have a diamond, unfortunately). Striking an edge with the other part of the rock produced little black flecks. The broken surfaces have a pattern that radiates outwards from a 1-cm diameter core, and some of the radiating lines look dark red almost rust-coloured and shiny when they catch the light. I am not a geologist, as I'm sure you can tell, but I have kept this thing because of its amusing shape, and because my companion that day told me with great seriousness that it was a meteorite (I know it's not though). All ideas very welcome. Thank you!

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    $\begingroup$ it is probably a fosilized coral of some kind,it is definitly not from space. $\endgroup$ – trond hansen Jun 19 at 17:17
  • $\begingroup$ en.wikipedia.org/wiki/South_Downs $\endgroup$ – trond hansen Jun 19 at 17:22
  • $\begingroup$ Thank you. Coral is one thing I had not considered, but it would explain the shape. $\endgroup$ – Sybilla Jun 20 at 17:13

It is a marcasite nodule. Marcasite is an iron sulphide, with the same chemical composition as pyrite but a different crystalline structure. Such nodules are often found in the chalk of SE England. These nodules are usually brown on the outside, but may be shiny and metallic inside.


Definitely not a meteorite (I was glad to hear you say you knew it wasn't) and not a fulgurite, as fulgurite doesn't have a radial crystal structure on it's interior. My initial guess was a fossilized burrow (very common) which would've been formed when a little critter way back in the past (millions of years ago) burrowed into the ground. Fossilized burrows are common and I suggest you Google those to see what you think. Look up "ophiomorpha" as it resembles that to me. It is VERY neat. I love it and I'm happy to hear you kept it all these years. It's a dandy rock.

  • $\begingroup$ Thank you, Rokman! I will look these up, I have never heard of them. $\endgroup$ – Sybilla Jun 30 at 15:15

The rock in the photo is a fulgerite, which is where a lightning strike has fused sand, clay or other soil components into a glassy mass. Fulgerites have no standard composition, as soils vary, but silica sand is a common component. Some examples are very old and could be called fossilised lightning strikes, other examples are more recent. I can't tell how old the illustrated example is.

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    $\begingroup$ fulgurite is hollow tubes of fused sand,it does not look like this is the case here. $\endgroup$ – trond hansen Jun 20 at 12:43
  • $\begingroup$ Well,if you don't want to believe me,that's up to you. Obviously fulgerites vary,because the composition of the soil varies and the strength of the lightning strike varies. $\endgroup$ – Michael Walsby Jun 20 at 13:02
  • $\begingroup$ en.wikipedia.org/wiki/Fulgurite $\endgroup$ – trond hansen Jun 20 at 15:04
  • $\begingroup$ I like the idea of fossilized lightning! I had never heard of fulgerites, so thank you for the suggestion. $\endgroup$ – Sybilla Jun 20 at 17:15

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