Origin of agate: a 300-year-old enigma

Quartz has been estimated to occupy around 12% of the earth’s crust and can be found in many forms, ranging from the massive, clear crystals of quartz and amethyst to the microcrystalline quartz that is to be found in jasper, agate, chalcedony, chert and flint. World-wide, the distribution of agate is not equal, but it can be found in every continent and probably exists in every country. However, only three countries extract enough agate for world export: Botswana, Brazil and Mexico.

Fig.1
Fig. 1. Empty gas cavities and agate amygdales in a block of Isle of Mull basalt. (Scale bar = 2cm.)

Agate is most frequently found in fine-grained, igneous rocks filling gas cavities (Fig. 1), but it can also be found in sedimentary limestone hosts (Fig. 2) and fossil wood (Fig. 3). The most common agates are the wall-lining and horizontally banded types (Figs. 4 and 5 respectively). Rapid identification of agate in the field relies on the natural translucency of a fractured sample, but final confirmation is supplied by examining a thin section under a polarising microscope (Fig.6). Agate and chalcedony show a fibrous structure, whereas the quartz in flint, chert and jasper is generally granular. Granular quartz can demonstrate regular banding, but this is not agate (Fig. 7). Nevertheless, it is the colours and rhythmic banding that makes agate the most recognisable of all gemstones.

Fig.2


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