Agate
Description
Agates form in voids within former rocks and many of the most spectacular examples come from open pockets created by trapped gas bubbles in basalt lavas. Ground water percolating through the basalts dissolves silica and other elements that are transported into the voids via fractures and cracks. The commonest form – banded agates – typically feature concentric shells differentiated by changing colours. These are derived from differences in the orientation of the microscopic fibrous crystals that make up the rock plus variations in trace elements within the crystals – variations as small as one part per million can change the colours significantly. The precise mechanisms of agate formation are still poorly understood. In the Brazilian agate of which this image is a detail approximately 8cm (3.5”) square, concentric bands have been freely distorted and frame irregular areas of white, crystalline quartz. The astronomer Kepler described minerals as examples of nature ‘at play’, and this image seems to me a perfect example of what he had in mind.