Agate

Big Photo

India
14.62 carats
© gemselect.com

Agate is a banded, concentric shell-like Chalcedony, sometimes containing opal substance. The bands can be multicoloured or of the same colour.
Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.

Agate Gemstones by Colour

This table shows the variety of hues this gemstone can be found in. Click on a photo for more information.
 
 
 
 
 
 
 

Agate Gemstones by Size

This table shows distribution of Agate gemstone sizes that are listed on this site. This can give a good indication as to the general availability of this gemstone in different sizes.
Contributed photos
Lightest:0.90 cts
Heaviest:356.1 cts
Average:22.33 cts
Total photos:58
Do you have a larger Agate? Why not upload a photo?
0.90ct to 36.42ct36.42ct to 71.94ct71.94ct to 107.46ct107.46ct to 142.98ct142.98ct to 178.50ct178.50ct to 214.02ct214.02ct to 249.54ct249.54ct to 285.06ct285.06ct to 320.58ct320.58ct to 356.10ct
General Information
A variety or type of:Chalcedony, which is a variety of Quartz
Varieties/Types:
Iris Agate - An iridescent variety of Agate.
Onyx - A monochromatic Agate with black and white banding.
Sardonyx - A monochromatic Agate with red/brown and either black or white banding.
Chemical Formula
SiO
 
2
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
Photos of natural/un-cut material from mindat.org
Agate Treatments
Much agate is dyed to give strong and commercially acceptable colours. The material is sawn and shaped before dyeing. Immersion in a hot sugar solution followed by immersion in concentrated sulphuric acid and heating gives black; a blue colour, once called Swiss lapis, is produced by immersion in potassium ferrocyanide and subsequent warming in a solution of ferrous sulphate to give a precipitate of Berlin blue. Chromium alum or potassium dichromate may be used to give green or
bluish green, and nickel compounds give a brighter, apple green. Impregnation with iron compounds and heating may give reddish brown and red colours. Yellow to greenish yellow is obtained by heating dry agate that has been treated with concentrated hydrochloric acid.
Physical Properties of Agate
Mohs Hardness6.5 to 7
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Specific Gravity2.60 to 2.64
Walter Schumann, Gemstones of the world (2001)
More from other references
Cleavage QualityNone
Walter Schumann, Gemstones of the world (2001)
FractureUneven
Walter Schumann, Gemstones of the world (2001)
Optical Properties of Agate
Refractive Index1.530 to 1.543
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Optical CharacterUniaxial/+
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Birefringenceup to to 0.004
Walter Schumann, Gemstones of the world (2001)
More from other references
PleochroismAbsent
Walter Schumann, Gemstones of the world (2001)
DispersionNone
Walter Schumann, Gemstones of the world (2001)
Colour
Colour (General)Banding of different colours. Colours are generally pale though varied; natural colours are green, yellow, red, reddish brown, white and bluish white, among others.
Michael O’Donoghue, Gems, Sixth Edition (2006)
TransparencyTranslucent,Opaque
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
LustreWaxy
gemdat.org, Management Team (2012)
Fluorescence & other light emissions
Fluorescence (General)Varies with bands: partly strong; yellow, blue-white
Walter Schumann, Gemstones of the world (2001)
Crystallography of Agate
Crystal SystemTrigonal
gemdat.org, Management Team (2012)
HabitMicrocrystalline aggregates
Walter Schumann, Gemstones of the world (2001)
More from other references
Geological Environment
Where found:It occurs in filling cavities, and individual bands are concentric to the external surface of the mass or nodule. Agate geodes are found in basic lavas and other igneous flow rocks and have probably been formed by silica deposition in cavities created by gases.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Further Information
Mineral information:Agate information at mindat.org
Significant Gem Localities
Peru
 
  • Huancavelica Department
    • Huancavelica Province
      • Ascencion District
        • Yauricocha Parish
UK
 
  • Scotland
    • Tayside
      • Montrose
        • Usan
[Specimen in the Natural History Museum, London]
USA
 
  • California
    • Siskiyou Co.
      • Klamath Mts
        • Cinnabar Camp
U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.
  • Utah
    • Juab Co.
      • Thomas Range
        • Topaz Mountain
U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.
Copyright © Jolyon & Katya Ralph 1993-2017. Site Map. Locality, gem & photograph data are the copyright of the individuals who submitted them. For more information please contact the . Gemdat.org is an online information resource dedicated to providing free gemmological information to all.