Agate

'... As e'en the silver on the cliff it shone; Of dark blue steel its columns azure height And the big altar was one agate stone. It seemed as if the air upheld alone Its dome...'

Esaias Tegnér, Fridthjof's Saga, Canto XXIII, Stanza 13

What is Agate?

Agate is a translucent variety of microcrystalline (cannot be seen with the eye) quartz.

Occurrence

Agate generally forms by the deposition of silica (the fundamental mineral in quartz) from groundwater in the cavities of igneous  (volcanic) rocks. The agate deposits in concentric layers around the walls of the cavity, or in horizontal layers building up from the bottom of the cavity. These structures produce the banded patterns that are characteristic of many agates. Some of these cavities are lined with crystals which are known as geodes.

 

Agate occurs in a wide range of colours, which include brown, white, red, grey, pink, black, and yellow. These colours are caused by impurities and occur as alternating bands within the agate.

The different colours were produced as groundwaters of different compositions seeped into the cavity. The banding within a cavity is a record of water chemistry change. This banding gives many agates the beautiful colours and patterns that make it a popular gemstone.

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