Agates
The blue/purple example on the left is known as eyed agate; this particular specimen comes from Uruguay. The white material between the coloured spheres is also agate. The red/yellow example on the right features concentric rings and comes from New South Wales in Australia. Both rocks have been polished to emphasise their details. They are on display behind glass in the Minerals section of the Natural History Museum in South Kensington, London.
Agate is a rock consisting primarily of cryptocrystalline silica, chiefly chalcedony, alternating with microgranular quartz. It is characterised by its fineness of grain and variety of colours.
Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Agate has also been known to fill veins or cracks in volcanic or altered rock underlain by granitic intrusive masses. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section.
Agates
The blue/purple example on the left is known as eyed agate; this particular specimen comes from Uruguay. The white material between the coloured spheres is also agate. The red/yellow example on the right features concentric rings and comes from New South Wales in Australia. Both rocks have been polished to emphasise their details. They are on display behind glass in the Minerals section of the Natural History Museum in South Kensington, London.
Agate is a rock consisting primarily of cryptocrystalline silica, chiefly chalcedony, alternating with microgranular quartz. It is characterised by its fineness of grain and variety of colours.
Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Agate has also been known to fill veins or cracks in volcanic or altered rock underlain by granitic intrusive masses. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section.