[Jewelry making - Article 00086] - Re: [Orchid] Holly Blue Agate Vs Chalcedony?

[Jim Small]

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Noel

    This is like asking what is the difference between a Ford and an
    automobile. All agates are forms of chalcedony, so chalcedony is the
    class of cryptocrystalline quartz which includes Holly Blue Agate
    (which is not blue at all, unlike Ellensburg Blue, for example). The
    identification "tree" begins with quartz, which can be crystalline
    or cryptocrystalline (and some mineralogists add a separate category
    for opal). Within cryptocrystalline there are several further
    categories, basically beginning with chalcedony as the purest form,
    working down to flint or chert as the most impure (purity being the
    relative percentage of quartz contrasted to impurities or non-quartz
    minerals).

HTH,
Jim Small
Small Wonders Lapidary
Church Hill, TN
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