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Rock Wedges, Human Made Or Biological?


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They are the imprints left behind by mineral crystals that have since weathered away. Pyrite, calcite and many other minerals can be selectively dissolved or oxidize away, leaving a very interesting defect in the host rock.

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I'm sorry, but I don't see any fossil here. This rock looks like a worn pebble of granite. Granite is an intrusive igneous rock, and most have a lot of feldspar minerals in them. Feldspars, when broken, fracture in a way that is almost always a perfect right angle. This produces something that looks like a bunch of miniature steps. If there was a large chunk of feldspar that formed in your rock, a collision with another river boulder could smash it, fracturing to make these "letters".

Another mineral like this is galena. See here.

Edited by PA Fossil Finder


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I would agree – neither man-made nor organic. Unfortunately that danged infernal slideshow doesn’t readily allow a single pic to be isolated and enlarged, but as far as I can see those are cubic holes and v-shapes from the corners of holes made by cubes. This sort of thing:


[image by Roel Tijskens]


[image by “Wrench” on Treasurenet forum]

Those ones are the result of minerals with a cubic crystal habit being eroded or tumbled out of the rock matrix. The most likely minerals for this would be fluorite, pyrite, magnetite, calcite (plus pseudomorphs), feldspar and – if you were in the right area (in Mexico you probably aren’t) – pseudomorphs after glendonite.

As PA Fossil Finder says, the cobble looks granitic (or at least igneous), so feldspar would be likely.


I keep six honest serving-men (they taught me all I knew);Their names are What and Why and When and How and Where and Who [Rudyard Kipling]

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Very instructional posting for those like me who are still learning about rock business, thanks Scilla, PA Fossil Finder, Painshill and

tlatoanitzin, great!!

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