The mysteries surrounding the formation of agates have long been debated and argued over for thousands of years, for just as long as they have been collected and fashioned into beautiful Jewellery! It has only been in the last 200 years, however, that serious investigative attempts have been made to explain their formation.
As trying to detail all the findings of these investigations here would take quite some time, instead I will explain the basics of agate formation.
Agate is chemically a form of micro crystalline quartz called chalcedony. Chalcedony in it’s purest form is colourless to pale grey, therefore Agate essentially is an impure form of Chalcedony.
The distinguishing features of agate is the coloured bands which are caused by natural impurities such as iron (red/brown colour) or manganese & cobolt. Agates occur mainly in small rounded nodules (lumps) within volcanic rock (lava). The agates essentially just occupied the sites of gas bubbles which formed as the lava cooled over millions of years. Research has shown that agates do not form in the final cooling stages of volcanic rocks, but only after complete cooling and burial of the flows to a depth of 100 to 200 metres. When this occurs, silica bearing solutions penetrate the lava and fill the bubbles with agate forming material.
The next stage is too complex to cover here, and to be honest people still debate today on exactly what happens when the impurities enter the bubbles, but the banding variation very slowly occurs deep below the lava surface.
After millions of years of weathering above ground, the agate nodules become exposed on the ground surface and this is when you can go and collect them.
To summarise, agate is impure chalcedony and is formed mainly in lava gas bubbles. The banding occurs over a very long time and colours are caused by impurities in the surrounding rocks, such as iron, which gives the very traditional red or brown colours.