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 6cm (2.5”) square, concentric bands completely line the cavity but their regularity has been disturbed in various ways, creating this image I called ‘Cosmic Egg’ when it first appeared on the computer screen.