Agate Formation

In his book “Agates” 1989 Harry Macpherson states: “the formation of lava flows and the formation of agates are not contemporaneous or even connected events.”

Molten lava contains gases held under pressure before being erupted on to the Earth’s surface. At the time of eruption, as the pressure is reduced, the space this gas occupies increases and gas bubbles form. The gases mainly involved include carbon dioxide, water vapour, sulphur dioxide, chlorine and even hydrogen sulphide. Many of these bubbles burst to the surface and the gas is vented but as the outer lava layer cools some of the gas bubbles are trapped.

Rich amygdaloidal lava with agate infill from Usan, near Montrose, Tayside

This can be considered the initial part of the formation process. These bubbles are called vesicles when the lava cools and later when filled with celadonite or agate become amygdales. Amygdaloidal (from the Latin “amygdula”, an almond) lava is so called because the original vesicular material contains almond-shaped cavities. As well as almond shaped these cavities can be totally misshapen, probably due to the flow effect within the flowing liquid lava, round, oval, almost flat or even bun-shaped. The viscosity of the fluid rock through which the bubbles are ascending may determine this shape. Heddle (1901) described amygdale shapes as round, rod-shaped, pear or balloon-shaped, axe-shaped and even wine-bottle shaped.

The cavities subsequently become filled with agate forming materials. Agates can form in fissures within rock called veins, or as long filaments similar to but more numerous than veins, called stringers. An example of this type of agate can be found at Burn Anne near Galston, Ayrshire. Agates can also form within sedimentary rocks as nodules that are the result of the replacement of a former mineral or some organic material such as wood or coral.

Amygdale in lava from Ethiebeaton Quarry, near Dundee Loose amygdale with red coating from Barras Quarry, near Montrose
Agatised coral (Lithostrotion sp. Carboniferous age from the Galdrons, Mull of Kintyre.

Agates do not appear to form in the final cooling phase of volcanic rock. It is only after complete cooling and burial of the flows to depths of at least 100 – 200 metres that they form (Macpherson 1989)

The description in the previous few paragraphs here is certainly a part of the more widely accepted initial theory of agate formation. From now on it becomes much more complex and scientifically controversial. There does not appear to be one unifying theory from this point.

It can however be said that in general they form from an amorphous deposit of silica-rich material which fills the cavity. It is thought that silica-rich solutions enter from outside and form gels inside or that solutions form outside and then move into the cavity. These solutions then deposit silica from the outer part to the inner part of the cavity over an indeterminate period of time.

Some recent work suggests that agates do not grow linearly but are subject to “stops and starts” Some researchers suggest that some of the oldest agates in the world have grown for the first 60 million years then stop for 210 million years and then grow again for the next 80 million years. The initial growth rate in these materials has been shown to be approximately 1 nanometre per million years (Moxon 2009). Others suggest that agates form in geological systems and appear to form in under 30,000 years (Donald Kasper, 2014). This researcher also suggests that agates form in Super Critical States with temperatures in the rage 374 – 575 degrees Celsius and pressures as high as 2kbar.

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The Uncertainties in Agate Formation

Given the structural and compositional similarities between agates from all parts of the world it has been hypothesized that agate formation occurs independently of any outside influences and that the banding as well as the colouring is largely chemically controlled. In other words, the variation in the texture and composition must result from the closed internal dynamics of the growth itself rather than any variable conditions outside the agate (Wang et al.,1995). The crystallisation behaviour that controls this compositionally simple mineral system is believed to be very complex.
Within the scientific community there is dispute as to the source of the silica, the temperature and pressure of formation and the mechanism of deposition.


Source of the Silica

Probably the biggest uncertainty in agate genesis is the source of the silica from which the agates are formed. The lavas within which the agates developed are generally very poor in free silicon dioxide. It has been proposed that the silica source could be from the local environment, hydrothermal activity, late magma deposition or a silica glass within the magma (Moxon, 2002). There are two opposing theories for this process, both with ample supporting evidence.

The first theory suggests that silica released from volcanic ash devitrifies and eventually is released in the form of a watery gel that then permeates through into underlying lavas via meteoric (atmospheric or rain) water. After the initial lava flows were laid down subsequent eruptions covered the basalts with a silica-rich rock called a welded ash-flow tuff. Alkaline or saline lakes later formed on the tuffs and freed silica from the volcanic ash. The resulting silica rich gel then moves downwards in the rock and enters the cavities.

The second theory suggests that agates form from a silica lump or gel within the magma that contains trace elements and water (Merino et al., 1995). Although some researchers believe that agate genesis is contemporaneous with the formation of the host rock, most have argued that it occurs up to tens of millions of years later. A third possibility is that the silica for the agate genesis could be mobilized from the surrounding wall rocks by hydrothermal activity (Moxon, 2002).

It has also been argued that if the cavity contained the silica gel at the beginning of agate genesis then as a result of contraction and loss of volume this amorphous silica deposit would require additional material (an estimated 20% of the total) in order to maintain a full amygdale of chalcedony. Therefore the agate amygdales would be reliant on a late input of percolating silica-rich solutions and that formation would not be a ‘closed system’. In addition, the similarity in the rare earth elements between agates and the parent lavas suggest that these elements are mobilized during syn-and post-volcanic alteration of the host rock.

There is also evidence suggesting that crystallization from fluids may occur with differing degrees of polymerization and that the observed alternating crystallization of quartz and chalcedony may be caused by variations in the degree of silica saturation of the silica-bearing fluids. Despite substantial support for this theory there is also ample evidence to the contrary and no one explanation convincingly accounts for all the data.

Crystallization of agates from an initial hydrous silica gel or glass lump has also been put forward as an alternative formation mechanism. These polymerized silica ‘lumps’ would contain trace elements and water and there is some evidence from high silica concentrations found in present-day hot springs that might support this theory.

One of the main problems with the hydrothermal fluid theory is that the initial crystalline deposit of chalcedony on the inside of the amygdale would be expected to block the ingress of further silica rich fluids. This would not be a problem with a ‘closed’ lump of polymerized silica within the magma as all the required silica would already be present.

Another variation on this theory has suggested that agates in volcanic lavas are xenoliths of marine chert because of the similarity in the 18Oxygen values between the two. These chert lumps would not be melted and reabsorbed by the lava but would be carried within it as melt drops and later transformed into agates (Fallick et al.,1985). In conclusion it is clear that there is not yet a final answer to the source of the silica as no one theory is compatible with all the evidence.


Agates have not yet been successfully formed under laboratory conditions. Many workers believe that agate formation is contemporaneous with the formation of the host rock. However even if this is the case it does not help answer the complex question of deposition. Most of the problems of deposition relate to the formation of the small scale microcrystalline quartzine- chalcedony bands and not the colour banding that can be seen with the naked eye.
It is thought that the zoning seen in the microcrystalline system is the result of a cyclical interplay between growth rate and diffusion rates at the crystal/solution interface. The high defect (Brazil-twin) density found and the impurities within the agates points to a rapid growth of silica from a strongly supersaturated solution probably with a non-crystalline precursor. This type of depositional process would be self-organizational and not dependent on external factors.
Other workers have attempted to explain the deposition using a chemically controlled method that explains both the twisted nature of the quartz fibres and the process that causes the visible banding in agates. This model assumes that agates form from amorphous lumps of silica within the magma and predicts that the fibre size changes periodically as seen in agates.

There are a number of other theories that attempt to explain deposition using percolating crystallization from fluids in hydrothermal systems. Each theory appears to explain agate deposition adequately but so far, unfortunately, none of them have resulted in agate-like patterns in laboratory tests. Conceivably, a better understanding of the timescale of formation is needed to construct a truly workable model of agate deposition.

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Temperature and Pressure of Formation

The temperature of formation of agates in an igneous environment is not known. It is a key point in understanding formation and studies undertaken have concluded that temperatures range between <50 degrees Celsius to >400 degrees Celsius. This wide range suggests one of two things: either agates form under a wide range of temperatures or the conclusions of such studies are inaccurate.

A direct estimate of the formation temperature of agates is difficult and the plethora of results from research into this topic does little to clarify the picture. Nevertheless the majority view suggests that genesis starts with polymerization of silica rich fluids at temperatures of more than 100 degrees Celsius.


An attempt has been made here to draw together the current thinking on the formation of agates and present it in as simplified a way as possible without losing too much of the detail. It is clear that there is no single accepted detailed explanation of how agates form. As mentioned above no researcher has yet managed to start with basic ingredients and create a whole agate nodule, exhibiting all the familiar agate characteristics, at the end of the experiment.

Extensive analysis of the literature suggests that no evidence has been found to date to demonstrate that agates are forming in real-time in basalts anywhere on the earth. I have to say I find this fairly hard to believe? However, a better understanding of the formation process would provide an idea of where to look. Conceivably, agates are developing in sub-sea geological structures that are not easily available for examination by land-based investigators.

Assuming no agates are forming on the earth today then the nearest situations that can be observed and studied would be locations where hot silica rich fluids are being brought to the surface and polymerized into silica.This appears to be happening in Yellowstone National Park in America and in parts of New Zealand and may be comparable to what took place in Scotland in the Devonian period at Rhynie in Aberdeenshire. Despite the uncertainty of the pressures and temperatures involved in agate formation at Rhynie we can say that the silicification probably occurred at atmospheric pressure and temperatures around 100 degrees Celsius.
We are therefore left with more questions than answers.

Perhaps agates form under a variety of differing conditions and processes and therefore there can be no one single unifying theory of formation? This certainly appears to be the case with the zeolite group of minerals, which also form within cavities in volcanic rock.
Some zeolites form at low temperatures by alteration of volcanic ash and larger pyroclastic material on the land surface, in freshwater lakes, shallow marine seas, saline, alkaline lakes and in deep-sea sediments. Very rarely do they form at low temperatures in vesicular volcanic rocks. Other zeolites form at high temperature from localized hydrothermal water in hot springs on the continents and ‘black smoker vents’ under the oceans and in broad regions heated by hydrothermal solutions or burial meta-morphism. At high temperatures, zeolites also form from the cooling of volcanic flows, late phases in pegmatites and miarolitic cavities in plutons, and as phenocrysts in basaltic magma.The final formation process may take place in a variety of the above conditions so that the resulting zeolite may have a complex formational history in a variety of geothermal environments. (Tschernich, 1992) Could the creation of agates also have a complex multi-formational origin?

 Occasionally within the basaltic lavas one can find a beautiful solid agate and then within a few inches another cavity filled with calcite, clay like material or even a geode. Could this observation in the field be explained by this multi-formational origin within different and changing geothermal environments?
The replacement of once living material held within sedimentary rocks and initially proceeding towards a fossilizing process can eventually create a fossil pseudomorph formed with the internal structures perfectly preserved in beautiful agate of differing colours depending on the structures present. This process is known as ‘silicification’ or ‘agatisation’

Silicification processes are seen in rocks all around the world and have resulted in agatised pine cones (Araucaria Mirabilis) from Patagonia, perfectly preserved wood structures from Arizona, colonial corals from Sumatra in Indonesia and Tisbury in Wiltshire and also in Scotland with the Carboniferous coral Lithostrotion from the lower reaches of the Firth of Clyde preserved in perfect detail. One example of the latter material is shown above. How does agate replace dead organic tissue if the theory of formation involves the cavity initially filling with a silica-rich solution? It must require solutions to enter the potential space occupied by this material as it is decaying.
External geothermal processes must be involved that vary with time, perhaps over a very long but variable length of time. This would be further supported by such simple observations as the initial formation of brown calcite crystals in many of the agates from Mull. The ambient external conditions change and then the calcite is eventually surrounded by agate.

These are just a few of the questions thrown up by a close look at the theories of agate formation.
Whatever the process of formation, the result after millions of years, can be an agate with beautifully differentiated and coloured banding. One cut of the diamond saw will reveal this unique pattern to the eye of the collector for the first time since it was formed.....however that was!

...and finally.....

Possible hydrothermal environment for agate formation?


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