Monday, December 03, 2012

Grape Agate

Grape Agate
Looks just like it sounds, like small clusters of grapes.  Some refer to this material as “eye agate” or “fish eye agate” or “bubble agate”.  If the balls of agate are really small it is sometimes referred to as “fish roe” agate.  The agate balls are usually red but they can be yellow or orange and range from <1mm to 1cm.  Often when the larger balls cut through they are layered with concentric rings of red, yellow, orange and a whitish blue agate and resemble an eye ball.  It is also known as red chalcedony.
This is one of those materials that can just be cleaned up and trimmed around the edge to produce a cabochon with great color and texture.
The ground is littered red for a couple of square miles where this material is found.  It is a site to behold as the ground in every direction is red from chunks and nodules of this material.
Complete nodules are hard to come by but worth the hunt.  They may or may not have the grape agate on the exterior.  When whole nodules are broken open to reveal the geode interior, it may have quartz, celestite, hematite, or barite crystals.  Fine needles of hematite found within the geodes but almost always break when the geode is opened.
This material is good for cabochons & display pieces.  Tumbling this material is good if you avoid the material with deep pits and divots.  Tumble only the exterior agate as the minerals on the inside are so soft they wear out leaving your tumbled stone with unpolished depressions and dull areas.
How the grape agate nodules form is not understood well.  They weather out of sandy shale “puddles” where they may have been a replacement of something in the sand.  The puddles are always rounded or oval shaped and weather out of the shale at a slower rate than the shale around them, as if the agate is holding them together.  The shale puddle always shows signs of being deformed as if the agate pushed down on the shale when it was settling.  But the agate didn’t form until after the shale was deposited.

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