The individual fibers of chalcedony and its subvarieties are not physically separable; they occur as parallel or near parallel aggregates or as spherules. The fiber direction is perpendicular to the layering while the banding is parallel to the free surface or cavity in which the chalcedony occurs.
The hardness of chalcedony is variable around 6.5; somewhat lower than that of more coarsely crystallized quartz which has a hardness of 7. The specific gravity also varies according to porosity, water content, and the presence of other substances. The normal range is 2.57 – 2.64. Chalcedony is about 90 – 99% SiO2. Brown and reddish varieties are higher in Fe2O3.
Chalcedony fractures easily across the banding to give a splintery surface with a waxy luster. Most chalcedony is somewhat pale and grayish in appearance.
Chalcedony is deposited in a variety of environments and deposition is commonly at low temperatures. Light colored material is often found as a late hydrothermal deposit or an alteration product in acidic or basic igneous rocks, tuffs and breccias. It is very common as crusts of vein or cavity fillings.
The most prized of the chalcedonies are the translucent green stones known as chrysoprase and the brown iridescent and colorful stones known as fire agates. Other important kinds of chalcedony include bloodstone, lace agate, sard, iris agate, onyx, and carnelian.
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