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Roger Weller, geology instructor                                      

wellerr@cochise.edu
Quartz Family Gemstones-by Audrey LaClair
                            
                          Agate
 

Agate is a part of the chalcedony family and is translucent.   It’s 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.

“Agate is characterized by its occurance, hardness, fracture, specific gravity, refractive index and luster” (http://www.mineralminers.com/html/agaminfo.htm)

photo by R.Weller
 

PHYSICAL PROPERTIES

• Agate Chemical composition: SiO₂
• Class: tectosilicate
• Crystal system: Hexagonal-R; 32 (trigonal-trapezohedral)
• Crystal habit: usually as banded or layered botryoidal masses with microfibrous structure
• Zoning: color banding and layers of inclusions are common
• Specific gravity: 2.57-2.64
• Index of refraction: 1.53-1.55
• Birefringence: weak, 0.005 to 0.009
• Hardness: 6.5 to 7
• Color: frequently colored by impurities and then may be any color, most agate used for ornamental purposes is colored by artificial means.
• Luster: dull vitreous to greasy
• Transparency: translucent, can be transparent or opaque
• Cleavage: Cryptocrystalline quartz shows no cleavage, although parting may occur along layers and splintery fracture can develope parallel to microfibrous orientation.
• Fracture: conchoidal to splintery
• Streak: white

From (http://www.mineralminers.com/html/agaminfo.htm).

References:

http://www.gemstone.org/gem-by-gem/english/agate.html

http://en.wikipedia.org/wiki/Agate

http://www.desertusa.com/mag98/april/papr/geo_agate.html

http://www.jewelrymall.com/stones/agate.html

http://rds.yahoo.com/_ylt=AgnevlLwdMeZ5oBDiB6u5mpXNyoA;_ylu=X3oDMTE2cWp0Z2FuBGNvbG8DdwRsA1dTMQRwb3MDNwRzZWMDc3IEdnRpZANDMjM4Xzgx/SIG=1238ni5ev/EXP=1133986169/**http%3a//www.infoplease.com/ce6/sci/A0802711.html

http://www.gemhut.com/agate.htm

http://www.bernardine.com/birthstone/agate.htm

http://geology.about.com/library/bl/images/blagate.htm

http://www.mineralminers.com/html/agaminfo.htm