[Jewelry making - Article 01034] - Re: [Orchid] Brittle Carnelian

[Wayne Emery]

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>     I have heard that most carnelian is heat treated, does anyone know
>     precisely how this is done? I'm wondering whether it's done to
>     maximize the color without regard for the integrity of the stone. 

    Carnelian is routinely heated to attempt to change the color from a
    yellowish-brown to a deeper rust color. This can be accomplished at
    fairly low temps, like 400-450 F. This causes a conversion of the
    limonite impurity in the chalcedony to convert to hematite, which is
    a rusty red color. It's a change in hydration state, basically, no
    re-ordering of atoms or color centers is involved, hence the low
    temps. The integrity of the stone is rarely affected, although
    trapped water, heated too quickly will turn to steam and cause an
    impressive explosion in your kitchen stove. 

>     The first one cracked when I let it wander onto a dry part of the
>     carbide stone for about a second. I cut another one very slowly,
>     using finer wheels and it turned out quite lovely. 

    The carbide wheel generated too much heat, which will immediately
    cause chalcedony to crack. Lower speeds, more lube, less pressure,
    finer grit, all can help. Also, the coarse grits ALWAYS cause
    sub-surface damage which is hidden until we get to the polishing
    stage. The cure is to use the proper succession of grits and
    pre-polish well. In essence, you must grind away the surface damge
    caused by the coarse grits, and it can go surprisingly deep in
    chalcedony....sometimes. 

    BTW, if you happen to live near Tenino, WA, you live an area once
    considered the source for the beat carnelian available. It still
    washes out of the banks of many of the creeks in fist-sized chunks
    and is easily collectible if you don't mind getting wet and muddy.
    Don't forget to ask th landowner's permission, lots of dairy farms
    there. Courtesy and packing out what you pack in goes along way in
    that part of the world. 

Wayne Emery
The Gemcutter
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