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About Minerals

Agate - Rich in Fiber

Sometimes chalcedony, including agate, is described as a fine-grained quartz but the real case isn't quite this simple. There are lots of clues for this. Arrowheads and other stone tools are harder and more durable when made from chalcedony than coarse quartz. On the other hand, coarse quartz is better to grind up as a concrete additive than chalcedony. The chalcedony causes various chemical reactions in the concrete, which can fail, while quartz is unreactive.

Chalcedony has a microscopically fibrous structure made of evenly spaced silica rods. Also, these fibers show a regularly alternating pattern of elongation-some parts being "length fast" and some parts "length slow." This means that in part of a particular fiber, light travels faster parallel to the long axis of the fiber. In other parts of the same fiber, the light travels slower parallel to the fiber length. This further implies that the silicon and oxygen atoms for some reason regularly twist or change in orientation as each fiber grew. Mineralogists are still trying to figure out why the fibers twist and what different forms of silica are intertwined with each other.

Via Osage Hills Gems 4/02