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World Agates

The photographs have been selected to pose interesting questions. I am in the process of writing a book entitled Studies on Agate where possible answers will be discussed in more detail. See the Studies on Agate link.
To contact me directly by email just click on the link: Contact Me

  • What is the world’s youngest agate? Chalcedony from the Yucca Mt., USA 9 Ma old is the youngest material that I have studied. However, chalcedony in some hot springs will be younger e.g. at Yellowstone Park, USA.
  • What is the world’s oldest agate? A difficult question because age of host and age of agate are not always the same. However, most agates form within a few million years of the host and the oldest agate host is the 3450 Ma Pilbara Craton, Western Australia (Fig. 13).
  • Can agates be dated? Generally the answer is no; at least not directly. However, a few agates do contain sufficient radioactive material to allow radio dating.
  • Are there other ways to date agate? The host rocks of agates in an igneous setting are often dated. If agate properties can be determined, then any links to the age of the host can be investigated. A plot of property vs age of host will show whether there is a relationship.
  • What properties show an age-related dependency?  Several properties do show this link. Agate is a mixture of moganite (a form of silica) and quartz. The moganite decreases with age while the crystallinity of quartz in agate increases with age.
  • Are there any other age-related properties? The defect site water (Si—OH) and density show a respective decrease and increase with age.
  • Is there any reason for the poor quality of agates from Cumbria and the Pilbara (Figs. 11 and 13)? These agates have been studied and have been shown to suffer from post-deposition heating.
  • Why do so many agates show a grey colour? Here, we could include Figs. 1, 3, 6, and 7. Colour is caused by metallic ions, particularly in the from of iron oxy-hydroxide. For some reason, the agates from Mt Warning, Rum, Romania and New Zealand have not included these colouring ions. Interestingly, these are all young agates: < 100 Ma.
  • Are specific colours and banding linked to particular areas? Yes and no! The Laguna agate (Fig. 2) with its large area of colourless chalcedony followed by a similar area of yellow can only be a Laguna agate. In an igneous setting, the Agate Creek agates do have an area-type lime yellow and shade of red (Fig. 5).
  • Do agates ever show large quartz crystals? Large granular quartz crystals are often associated with agate. Agates from some regions are more prone to this than others. When large crystals form, they are usually around the centre of the agate (Figs. 9 and 13).
  • Are agates found outside an igneous setting? Agates do form occasionally in limestone (Fig. 15) and can also be found in fossil wood (Fig. 14).
  • How are agates formed? The hardest question of all. The answer can only be supplied when science can fully answer  the following: What is the temperature of agate formation? What is the initial form of the deposited silica? How does silica get into the gas cavities? What is the process of crystallisation that allows the bands to form? Apart from isotope evidence to suggest that agates form at temperatures < 100oC, the answers have not yet been fully established;
  • Why has the agate in Fig. 4 taken on this squashed appearance? In an igneous setting, agates are mainly the result of a gas-cavity infill. The molten lava flows and the gas bubbles can take up some weird shapes with the flow movement. The cavity is later filled with agate.
  • Why is the agate in Fig. 8 surrounded by host rock as part of the nodule? These agates are formed in a rhyolite host rock and unusual conditions prevailed to create these agates.
  • Why do some bands form horizontally? It is assumed that these bands are due to the influence of gravity (Figs. 7, 8 and 9). However, this type of agate is region-related: found in some areas but not others. Note that the Scottish sample clearly shows at least two lots of silica input: silica that is iron oxide free and silica that contains iron oxide.
  • The agates in Figs. 6 and 10 look different to most of the others? These are vein agates that have formed as a fissure infill. A line through the centre shows the two halves as a mirror image. Sometimes, the agate fills the whole of the cavity as  one  infill without creating the mirror image sequence.

Agate photographs (Scale bar = 2 cm)
mt. Warning Agate  Laguna Agate

Fig.1 Mt. Warning, Australia  Fig.2 Laguna, Mexico

 

Isle of Rum Agate     Bobonong Agate

Fig. 3 Isle of Rum, Scotland Fig. 4 Bobonong, Botswana

 

Agate Creek Agate      Gurasada Agate

Fig. 5 Agate Creek, Australia Fig. 6 Gurasada, Romania


Mt. Somers Agate    Thuringia Agate

Fig. 7 Mt Somers, New Zealand   Fig. 8 Thuringia, Germany

 

East Midland Valley Agate    West Midland Valley Agate

Fig. 9 East Midland Valley, Scotland Fig. 10 West Midland Valley, Scotland

 

Cumbria Agate   Lakes Superior Agate

Fig. 11 Cumbria, England  Fig. 12 Lakes Superior, USA

 

Pilbara Craton Agate

Fig. 13 Pilbara Craton, Australia

 

Fossil Wood Agate     Fairburn Agate

Fig. 14 Fossil wood, Utah, USA Fig. 15 Fairburn, South Dakota, USA

 

The photographs show the similarity and differences between agates found across the world. The study of agates would not have started without the generous donation of samples from Rob Burns, Jeanette Carrillo, Roger Clark, Ciprian Constantina, Nick Crawford, Brad Cross, Martin Dartsch, Dick Dayvault, Robin Field, Gerhard Holzhey, Brian Isfeld,  Paul Hoskin, Herbert Knuettel, Reg Lacon, Brian Leith, Ian Lennon, Maziar Nazari, Dave Nelson, Leonid Neymark, John Raeburn, John Richmond, Will Smith, Vanessa Tappenden, Bill Taylor, Bill Thiess, Bill Wilson, Scott Wolter and Johann Zenz.
Agates have been used from: Igneous hosts (the age of the host in million years –Ma). Initials are the donor.  Yucca Mt, USA – 13 Ma (LN);  Mt Warning, Australia  - 23 Ma (J Ri); Chihuahua, Mexico- 38 Ma (BC); Cottonwood, Texas, USA- 37 Ma(BC); , Ellensburg, Washington, USA- 43 Ma (PH); Las Choyas, Mexico- 45 Ma (BC, JC)  Khur, Iran -50 Ma (MN);  BTVP -60 Ma (RL); Gurasada , Romania - ~ 60 Ma (CC); Mt Somers, New Zealand- 89 Ma (RB, VT)   Bobonong, Botswana-180 Ma (HK)   Nova Scotia, Canada – 200 Ma (BI);    Agate Creek, Australia-275 Ma (NC); Thuringia Forest, Germany – 285 Ma (GH); Buxton, Derbyshire – 311 Ma ( TM); Northumbria -391 Ma (NC)   East  Midland Valley, Scotland – 412 Ma (BW, BL); West  Midland Valley, Scotland -412 Ma (J Ra);  Cumbria, England – 430 Ma (RF); Cumbria,  England- 452 (RF);  Northern Territory, Australia- 513 Ma (JRi); Maydena, Australia- Cambrian (JRi); Lake Superior, USA  - 1100 Ma (SW, BC); Killara, Australia- 1840 Ma (DN); Maddina, Australia – 2720 Ma (DN); Warrawoona, Australia – 3480 Ma (DN).
Sedimentary hosts (these are rarely radiodated and the ages are the Geological Period).
Nebraska Blue, Nebraska, USA-Eocene (RC); Lyme Regis, England – Cretaceous (IL);  Dulcote, Mendips, England- Triassic (JZ); Fairburn, USA- Permian (RC); Union Rd, USA- Lower Carboniferous (BT) ; Paint rock, Tennessee, USA –Ordovician (WS); Fossil wood Utah, USA – Jurassic (DD).