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. Typically, they are formed of concentric bands, often with an area of quartz in the centre, but in this specimen this has been reversed: very narrow bands of agate line the cavity, and this exquisite agate formation floats within a large area of quartz. I find minerals from many sources, but this specimen came via an unusually global route: mined in Brazil, it was bought in a shop in Taiwan by a Danish professor of architecture who took it home and posted it to me from Aarhus!