The structure of the crypto-crystalline quartz varieties is fine-grained and fibrous. Careful microscopic examination of chalcedony and its sub-varieties shows a fibrous structure with the direction of the fibres perpendicular to the direction of layering and to the free surface. It is not possible physically to separate the fibres as it would be in the asbestos-like minerals; the fibres are only a few microns in diameter and up to several hundred microns in length. They occur as parallel or sub-parallel bundles, sometimes divergent, or as leafy aggregates. In some chalcedony it may not be possible to detect the fibres but a clue to their presence is given by optical extinction parallell and perpendicular to the banding direction. Fibres are usually elongated at right angles to the c-axis and may be helically twisted along the axis of elongation; this may give a periodic variation in birefringence along the length as in iris agate. Chalcedony fractures with some ease in directions across the banding and parallel to the fibres producing a splintery surface. It is porous to a greater or lesser extent and contains isolated pores or thread-like openings parallel to the fibres. This allows dyeing with a variety of colours. There is a slight variability of hardness, and at about 6l/2 on Mohs' scale chalcedony is a little less hard than single crystal quartz.
The chemical composition of chalcedony is 90-99% silica with some water and Fe and Al oxides as impurities. The higher values of silica content are found in the light or milky material; reddish and brown colours have a high content of Fe2O3. Hydroxyl (OH) has been found to be present and may occur in the interior structure of the fibres; much water can be lost by heating up to 100°C. Loss of water produces a lowering of refractive index and specific gravity.
from "Quartz" by Michael O'Donoghue, 1987. Copyright © 1987.
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