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The rare and unsual forms of agates are those obtained from the limestones
and shales, the forms of sedimentary rocks formed by anicent seas, and
their origins have perplexed the agate experts. There are at least three
different types of concretions that had the cavities filled in with agates.
The concretions was formed by expanding of fossils (or other nodules) by
water or evaporate minerals such as gypsum that would later be dissolved
to form cavities or the agate to fill in while the water would form the
cavities dissolving away any soft minerals within the chert nodules to
be later filled in by agates. Lastly, the hollow parts of the fossils wouldbe
filled in by agate. It appeared that the limestones and shales were enriched
in the minerals that would provide bold colors in many sedimentary type
agates.
Agates of United States and Canada:
Agates of Europe:
Agates of South America:
Agatized Fossils:
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