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