|Type of Rock:||Microcrystalline silica|
|Best Places:||Stenness beach, Eshaness|
|Associated with volcanic rock.|
Agate is a microcrystalline variety of silica, characterised by its fine grain and bright colour. Although agates may be found in various kinds of rock, they are usually associated with porous volcanic rocks, typically found in the north-west Mainland of Shetland, particulary at Eshaness and also on Papa Stour.
The process of agate formation is not fully understood, but begins as lava erupts from the earth. A drop in pressure as the lava emerges allows gasses to bubble out of the molten lava, just like bubbles in a fizzy drink. Many of these bubbles escape to the surface but some are trapped as the lava cools, creating vesicles (cavities) in the solid rock. These are sometimes twisted into odd shapes (amygadales) by the movement of the solidifying lava.
The next stage in agate formation involves water. Over thousands of years, water carrying silica and other minerals in solution, percolates through the buried layers of porous volcanic rock to fill the vesicles. These minerals come out of solution and fill the cavities to form agates.
Occasionally, changing chemical and physical conditions produce concentric layers different colours and a banded agate is formed. Agates are very resistant to erosion, and survive long after their host rock has weathered away so they can ocassionally be found lying loose among pebble beaches.