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A bird’s-eye view of a fortress shows different straight lines or circular segments, which often meet at guard towers. In an agate, the birds-eye view provided by a cutting a slice from the agate often reveals a series of curved segments, straight segments or circular segments, which join together. (In agate, there is often a very thin line where the segments meet.) Such a pattern of curves, straight lines and circles reminded early agate collectors of the bird’s-eye view of a fortress and called the resulting pattern ‘fortification banding’.

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