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Need help in understanding the cause of white and brown formations on the skin of the agate.

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asked Dec 7, 2015 by arbayowling (600 points)

2 Answers

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Formations are usually iron when they are brown. White could be Quartz. Mustard yellow is limonite, also an iron side effect. Some agates have matrix which hasn't eroded away and that is rhyolite and other volcanic rock.
answered Feb 27, 2016 by Weasel (58,980 points)
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You know those might be tubes. The agate is formed within air bubbles formed in cooling lava. Sometimes organic material is replaced with silica and then forms within the agate after the organic matter decays. Reading up on the theories about formation of agates will show how several different ways for different theories account for tubular formations. Some of them say natural fissures in the rock allow silica rich fluid to flow into the air pocket and fill and evaporate forming layers. The fluids have different colors because of different minerals forming layers of different colors. Over time some of those fissures fill with fluid and form agate or Quartz or really any mineral that fills the matrix.
answered Mar 1, 2016 by Weasel (58,980 points)