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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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closed as off-topic by user12525, Fred, Gimelist, trond hansen, BillDOe Sep 1 at 20:02

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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
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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.

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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.

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