GEOFF TOMPKINSON / SCIENCE PHOTO LIBRARY GEOFF TOMPKINSON / SCIENCE PHOTO LIBRARY
Polished agate slices. Polished slices of coloured agate. Agate is a type of chalcedony, which is itself a microcrystalline variety of the mineral quartz (silicon oxide). Agate is formed by precipitation from watery solutions within cavities in lava rocks. The bands of colour seen in these slices represent the edges of successive layers of mineral deposited during formation of the crystal. The presence of impurities can give a wide range of colours distributed in regular bands or zones, in clouds or in dendritic (branching) forms.
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