MICHAEL BARNETT / SCIENCE PHOTO LIBRARY MICHAEL BARNETT / SCIENCE PHOTO LIBRARY
The cut and polished surface of a nodule of agate, a cryptocrystalline variety of the mineral quartz (silicon oxide). The presence of impurities can give a wide range of colours which can be distributed in regular bands or zones, in clouds or in dendritic forms. The banding is usually in parallel lines of different colours, sometimes straight, sometimes undulating or concentric. The bands represent the edges of successive layers of deposition from solution in cavities in rocks. Fine specimens of agate are found in Brazil, India, Uruguay and Germany.
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