The common pattern in agates, which consists of concentric bands of varying colour and width that reminds people of crenulations in a fortress wall. Such a band has three structurally different parts. The first layer formed consists of tiny quartz fragments or crystallites (nanometre to tens of nanometres in size) where the crystal axes are essentially indistinguishable. The second layer is the chalcedony layer composed of stacks of quartz crystal fragments, from hundreds of nanometres to micrometer in size, where the crystal axes are distinguishable. The final layer of a fortification band is the coarse or euhedral quartz, which is tens of micrometers to centimetres in size and have the “c-crystal” axis roughly perpendicular to the fortification band.
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