[Jewelry making - Article 00015] - Re: [Orchid] Holly Blue Agate Vs Chalcedony?

[The Doctor]

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    Technically, not a thing, Noel. Holly blue agate IS chalcedony.
    Sard, sardonyx, chrysoprase, agate, jasper, chrysocolla, bloodstone
    and myriad others (such as flint) are also varieties of chalcedony.
    Basically, chalcedony is a cryptocrystalline (invisible to the eye)
    form of quartz. The differences among the different varieties are in
    the other trace minerals present when they formed, along with the
    various geologic events that formed them. 

    The number of these varieties is staggering. The number of different
    types of agate and jasper alone boggles the mind. Most are typically
    named after the location where it was discovered, or the person who
    discovered it. Montana agate, Biggs picture jasper, Lake Superior
    agate, etc. are examples of location names. Others might be named
    after inclusions, such as dendrites. Banded agate is named after the
    banded structure. Geodes can have cryptocrystalline banded chalcedony
    near the outside of the formation, microcrystalline formation closer
    to the center, and very large individual crystals lining the inside
    (think geode cathedral). 

James S. Duncan, G.G.
James in SoFL


 
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