I have been interested in the problems of agate genesis for over thirty years and this Web site looks at various aspects of this fascinating mineral. My particular interests are agate genesis and characterizing agates from around the world. Over the past eight years the work has been carried out at the Department of Earth Sciences, Cambridge University, England and funded by The Leverhulme Trust for three years.
What is agate?
Quartz is a major component of the earth’s crust and it is around 12% by some estimates. The quartz that is presently on the earth’s surface is known as low (or α-) quartz; high (or β-) quartz only exists at temperatures > 573oC. Low quartz reveals itself in many forms and the collector will be well aware of rock crystal (colourless), amethyst (purple) and smoky quartz (brown). Although the colours are different they are all characterized by the same crystal structure and they can grow into very large (macrocrystalline) crystals. Agate and flint seem far removed from rock crystal; indeed fractured agate and flint appear to be more like the amorphous glass. Nevertheless, agate, flint, chert and chalcedony are all forms of microcrystalline quartz and need microscopic techniques to show their true crystallinity. The quartz microcrystalline family is an important group of minerals that can often only be classified under a polarizing microscope. Here, flint and chert show a granular structure while agate and chalcedony reveal a fibrous texture. Agate is simply chalcedony with bands.
Where are agates found?
I have been able to obtain a suite of agates that have been sent by knowledgeable collectors from around the world. Agates occur on every continent and I have no doubt that they can be found in every country. However, the distribution of agates world-wide is uneven. The greatest diversity of agate types, colour and distribution is found in Australia, Brazil, Germany, Mexico and the USA. Outside these areas, agates are hard won. There are several areas in the UK where agates are to be found: Derbyshire, Mendip Hills, Cumbria and Northumbria although the pride of British agates is in the Midland Valley, Scotland.
The agates that follow are really working agates in that samples are removed for scientific testing. I do have one or two prize pieces and there is a reluctance to do more than drill out powders from these. The agates are shown in order of the age of the surrounding host rock.