Saturday, May 26, 2012

OBJECT #10: Agate

I can’t remember how I obtained this slice of agate.  My basement contains hundreds of rocks and minerals and it’s easy to lose track of where they all came from.  My husband is the main rock collector, so that makes me “once removed” from parts of the collection, giving me an excuse for not being able to remember where they came from.

    An agate is actually considered to be a type of gem stone, often classified as chalcedony, which is in the quartz family (SiO2).  The name “agate” was given to these rocks by an ancient Greek named Theophrastus, who named them after the river where he found them:  the Achates.

    Many agates probably started out as hollow bubbles in lava.  As the lava cooled, hot mineral-rich water seeped into the bubbles.  As the solution cooled, the minerals precipitated out and formed crystals.  The color of an agate was determined by what kind of minerals were in the solution.  Some of the minerals cooled into very fine-grained rock, with particles so small that the texture of the rock is smooth and glassy.  Sometimes there was enough mineral soup to fill the entire empty cavity, but in some agates the minerals ran out before they reached the center, leaving an empty space.  Often, the edges of the central empty cavity are ringed with large crystal formations. 

    Some agates are found as “geodes,” spherical rocks that must be cracked open to reveal the gems inside.  (Mexico has some really good geode sides.  There are places were thousands of small geodes can be easy collected.)  Some geodes turn out to be “duds” and aren’t much to write home about.  Others will be found to contain crystals that are quite attractive.   One of the largest geodes ever found was in Brazil. It weighed 35 tons and was filled with amethyst crystals (a type of quartz that is often light purple in color).

    One type of agate is not spherical, but occurs in long stripes.  In this case, the mineral-rich silica water filled in long, narrow cracks in the cooling rocks.  They are simply called striped agates, or banded agates.  They have the same beautiful multicolored patterns as the round ones.

    Agates come in lots of colors, but most often in shades of red and brown, gray, or blue.   Agates are used primarily for decorative purposes, such as jewelry or crafts such as book ends.  The only practical use for agates that has ever been documented is burnishing leather (a few places in Asia).  There is a church in Oregon where thin slices of local agates were used as panels for the stained glass windows.  I guess that’s sort of a practical use, but it’s decorative as well, so we could count it in both categories.  You can see from one of my pictures how light will go through these thin slices of rock.  This makes sense when you remember that silica, the main component of agates, has the chemical formula SiO2, which is the same formula for glass (and sand).

    There are lots of places online where you can buy geodes if you want the thrill of cracking open your very own never-before-seen specimen.  If you want a chance to be creative and create your own personal agate design, check out the agate craft idea on my website.  Just click on Free Downloads, then on Earth Science.

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