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Roger Weller, geology instructor                              
Quartz Family Gemstones-by Audrey LaClair

Agate is a part of the chalcedony family and is translucent.   Itís chemical structure is found to be identical to chert, jasper, petrified wood, tigereye, and flint.  It is also commonly associated with opal. 


photo by R. Weller 


Most agates form in cavities or nodules in eruptive rocks or from ancient lavas.  It is then filled, either partially or fully, by a siliceous matter and then forms layers on the walls.  When cut transversely, these agates reveal parallel lines, which give a banded appearance to them.   While the agate is being crystallized, the colored bands are not disturbed in any way.   Since agate is porous, it is often stained or dyed.


photo by R.Weller

Idar-Oberstein, Germany is where most agates are cut and stained.  Agate is most commonly found in Brazil, Uruguay, and Mexico.  In the USA, one can find agates in Washington, around Lake Superior, Colorado, Oregon, Arizona, Idaho, Montana and California.  


Agate is usually used as a gem or ornamental.  The word Agate comes from the Greek language from a stone found in the Achates River in Sicily.  The river is known today as the Drillo River.


ďAgate is characterized by its occurance, hardness, fracture, specific gravity, refractive index and lusterĒ (



photo by R.Weller


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