ters cassidoine), a
variety of native silica, often used as an ornamental stone. The present
application of the term is comparatively modern. The "chalcedonius" of
Pliny was quite a different mineral, being a green stone from the
copper-mines of Chalcedon, in Asia Minor, whence the name. There has
been some confusion between chalcedony and the ancient "carcedonia," a
stone which seems to have been a carbuncle from Africa, brought by way
of Carthage ([Greek: Karchedon]). Our chalcedony was probably included
by the ancients among the various kinds of jasper and agate, especially
the varieties termed "leucachates" and "cerachates."

By modern mineralogists the name chalcedony is restricted to those kinds
of silica which occur not in distinct crystals like ordinary quartz, but
in concretionary, mammillated or stalactitic forms, which break with a
fine splintery fracture, and display a delicate fibrous structure.
Chalcedony may be regarded as a micro-crystalline form of quartz. It is
rather softer and less dense than crystallized quartz, its hardness
being about 6.5 and its specific gravity 2.6, the difference being
probably due to the presence of a small amount of opaline silica between
the fibres. Chalcedony is a translucent substance of rather waxy lustre,
presenting great variety of colours, though usually white, grey, yellow
or brown. A rare blue chalcedony is sometimes polished under the name of
"sapphirine"--a term applied also to a distinct mineral (an
aluminium-magnesium silicate) from Greenland.

Chalcedony occurs as a secondary mineral in volcanic rocks, representing
usually the silica set free by the decomposition of various silicates,
and deposited in cracks, forming veins, or in vesicular hollows, forming
amygdales. Its occurrence gives the name to Chalcedony Park, Arizona. It
is found in the basalts of N. Ireland, the Faroe Isles and Iceland: it
is common in the traps of the Deccan in India, and in volcanic rocks in
Uruguay and Brazil. Certain flat oval nodules from a decomposed lava
(augite-andesite) in Uruguay present a cavity lined with quartz crystals
and enclosing liquid (a weak saline solution), with a movable
air-bubble, whence they are called "enhydros" or water-stones. Very fine
examples of stalactitic chalcedony, in whimsical forms, have been
yielded by some of the Cornish copper-mines. The surface of chalcedony
is occasionally coated with a delicate bluish bloom. A chalcedonic
deposit in the form o