I found this rock in a tropical climate river in Bolivia (specifically Ivirgarzama). It seems to have 5 or 6 layers.

  • 1st layer is a slightly course matte rock layer
  • 2nd layer is a more glossy rock layer
  • 3rd layer is like the second layer but lighter
  • 4th layer is this dark red coat rock layer
  • 5th layer has a matte finish to it more sandy-grainy looking
  • 6th layer definitely has a sandstone look to it

It's roughly 5 cm (2 in) high, 4 cm (1.5 in) across, and 15 cm (6 in) in circumference. I can scratch it with a knife blade so roughly a 5.5 in hardness. If possible could you also explain how this occurs in nature?

side of sample

end of sample

sample in hand

measuring circumference

  • 1
    Can you scratch the glossy layer with a knife or a file? – Armondo Villaescuza May 10 at 6:49

Looks like a cylindrical chert concretion, which was originally hosted in limestone or chalk or something like that.

Here are some pictures of similar rocks.

You can try dropping some vinegar on the white parts and see if it fizzes, and try scratching the brown shiny part with a knife (it will not scratch) for a confirmation.

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