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The following small egg-like rock was found laying on the ground in Tampere, a city in southern Finland. It weights about 950 grams, has a volume of about 360 cm3 and thus a density of about 2600 kg/m3. It's longest diameter is about 140 mm. The outer grey layer is about 2 to 3 mm thick. The core looks like ironstone. The hardness of both the outer layer and the core are more than 2.5 and less than 5.5 on Mohs scale. The flattest side of the rock has a thin layer of some black soft substance, maybe asphalt. A compass or strong magnet did not react to the stone in any way and neither did a geiger counter. The rock looks like a fossil egg but I guess there is a more probable explanation. Could you be able to identify what this rock is and in which kind of a geological process such rocks can be formed?

(Click images for full-size versions.)

  1. The egg stone from the top The rock from the top.

  2. The egg stone from the side The rock from the front.

  3. The egg stone from the bottom The rock from the bottom.

  4. The egg stone from the back The rock from the back.

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closed as off-topic by Spencer, Fred, Jan Doggen, Semidiurnal Simon, trond hansen Aug 7 at 9:55

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions about rock identification requests are off-topic. For more information, see the announcement on meta." – Spencer, Fred, Jan Doggen, Semidiurnal Simon, trond hansen
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ If you don't mind, can you break it in half with a hammer and show the pictures of that? 2600 density is not an ironstone. It's also not a fossil. Seems like an igneous rock with loads of tarnish on the exterior. $\endgroup$ – Gimelist Aug 20 '16 at 0:57
  • $\begingroup$ @Michael Thanks for the suggestion! I am interested to cut it in half although it is a nice piece of rock as it is. What kind of tools would you recommend apart from the hammer? Is a hacksaw best for the job or should I wait to find a proper circular saw? I would like to make the cut as clean as easily possible. $\endgroup$ – Akseli Palén Aug 24 '16 at 8:56
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    $\begingroup$ You would generally need a diamond rock saw or possibly a tile saw to cut the rock. Most of these would also be liquid cooled, water or oil. I wouldn't necessarily expect that the inner hardness will be like the outer hardness. $\endgroup$ – Friddy Aug 24 '16 at 18:45
  • $\begingroup$ Adding a comment to note that this is being flagged as off-topic now, three years after it was asked, because rock identification questions have now been decided to be off-topic on this site. This should not be seen as any reflection upon the asker, who asked this question in good faith in 2016. $\endgroup$ – Semidiurnal Simon Aug 6 at 23:24
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This one is a simple concretion. The interior is mudstone, it is rich in iron, lending to it's orange-brown color. They are dense, very common and often made from a small organic material (a piece of a plant, shell or animal) rolling around in sediment and accreting material in a concentric pattern. Hence, when it hardens into rock and thus breaks, it breaks in concentric layers like that of an onion.

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    $\begingroup$ rolling around is not technically correct, it is caused by electrochemical interaction between dissolved minerals. $\endgroup$ – John Jun 25 at 18:53
  • $\begingroup$ That's neither here nor there. I stand by my comment to help the lay person understand the mechanics of sediment accreting to form the shape of a concretion. $\endgroup$ – Rokman Jun 26 at 4:54
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    $\begingroup$ Concretions in paleontological context can also be formed by chemical processes like mineral binding by local self-eutrophication of an organism through taphogenesis $\endgroup$ – JulPal Jun 26 at 12:36
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I own this stone which was given to me as dinosaur egg (I'm vertebrate paleontologist). It weighs 2.9 kilograms and shows something shell-like. It's no dinosaur egg or something like this, it's a simple iron rich concretion which shows some shell-like erosion structures. enter image description here

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There are four possible sources of this type of shapes of rock.

  1. Meteorite: Meteorite during its descent in the Earth's atmosphere get heated up and metls (depending upon the chemical composition) take egg shape. One more clue one can look for is wrinkle shapes (similar to when one blows air on liquid surface) on its unbroken surface.
  2. Volcanoes: During volcanic explosion lave which is in melted form assumes drop shape (egg shape) when thrown far distance. On falling to the ground, sand or whatever material present on the soil will stick to it and give it two layered structure. In this case also one can look for wrinkled surfaces if they are present.
  3. Fossilised egg: This is a possibility. The rock looks too elongated to be fossilised egg but that does not rule-out the possibility.
  4. Stone-age tool: This is a possibility. The shape looks like stone age tool but its layered structure and relatively fragile appearance of the top layer makes it less likely.

If it is of volcanic origin then there is not much commercial value but if it is meteorite, fossilised egg or stone-age tool then it can have good value. In case of meteorite, breaking it in parts will not reduce the value but for fossilised egg or stone-age tool breaking it in parts will reduce its commercial value.

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    $\begingroup$ As a general layman on the topic, I'm a bit surprised erosion isn't more of an option. What rules that out? $\endgroup$ – JeopardyTempest May 18 '18 at 1:13
  • $\begingroup$ (Obviously in some senses not a true "source"... but can explain rock shape and characteristic all the same) $\endgroup$ – JeopardyTempest May 18 '18 at 1:14
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    $\begingroup$ I am not very sure but four reasons I thought makes the rock unlikely candidate for erosion. (1) The shape is unusual for water erosion. Erosion makes shapes which are more circular/spherical (2) Erosion makes surface much smoother. (3) It is very unlikely to have layers for rock which is undergone erosion. The top layer of the rock in the photograph appears too weak to withstand erosion (4) Though Akseli did not specifically mention, I assumed that he has found this particular rock unusual for its surroundings (otherwise Akseli would have posted pictures of bunches of rocks). $\endgroup$ – Harish May 23 '18 at 8:12
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    $\begingroup$ Is there a reason you ignore the much more likely answer of concretion? $\endgroup$ – John Jun 25 at 18:50
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    $\begingroup$ (3) This is definitely no fossilized egg... No pore structures or calcite prisms are visible. The most fitting egg type would be Elongatoolithus but this can be ruled out due to the pints I mentioned before. (4) what kind of tool would this be? I saw lots and lots of stone tools. They are typically made of of flint, obsidian, or any material with conchoidal fracturing of the surface (produces sharp edges). So why should the stone age people use a blunt stone for this? As a fossil hammer? I can show you some tools of my collection and they look really different. $\endgroup$ – JulPal Jun 26 at 12:31

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