variety. It has been described as an onyx with a deep brown
layer under a stratum of bluish white, so that the dark color
underneath is seen through it. The Italian nicolo is represented as the onyx covered by bluish white spots encircled by
milky zones, and by cutting out the spots an artificial kind
is obtained. Nicolo is found in Bohemia and the Tyrol.
Chalcedony. — This mineral affords another illustration of
the confusion introduced into the classification of precious
stones, in consequence of the want of agreement among"
mineralogists. Westropp, with others, groups a large number
of varieties under the head of chalcedonic quartz, and says
pure chalcedony is colorless or pale horn color, but when
tinted with iron or other substances, it forms an almost endless
variety of sards, agates, carnelians, plasmas, etc. King defines
it as a translucent variety of quartz, mixed with opal, and of a
waxy appearance ; Dana calls it a variety of quartz of a waxy
lustre, transparent to translucent, varying in tints from white t&
black ; while other writers represent chalcedony as a variety of
quartz with alternate stripes of white and gray, as well as a
term for all stones of the agate kind.
Many of the most beautiful silicious minerals are transparent chalcedonies of emerald green, purple, red, and blue
tints. It is never found in regular crystals, and has very little
lustre, but it is well adapted for engraving, for which it has
been used from very remote times. A white chalcedony, with
minute blood-red spots, has sometimes been called " St.
Stephen's Stone."
Sapphirine is the name given by lapidaries to blue chalcedony, but this is distinct from the species of the same name,—a
silicate of alumina, magnesia, and iron, of a pale blue or green
color, with a hardness and specific gravity superior to those
found in quartz.
