Amethyst
Amethyst
is a violet variety
of quartz often used in jewelry. The
name comes from the Ancient Greek
ἀ a- ("not") and μέθυστος methustos
("intoxicated"), a reference
to the belief that the stone protected
its owner from drunkenness. The
ancient Greeks and Romans wore amethyst
and made drinking vessels of it
in the belief that it would prevent
intoxication. It is one of several
forms of quartz. Amethyst is the
traditional birthstone for February.
Structure
Amethyst
is a purple variety
of quartz (SiO2) and owes its violet
color to irradiation, iron impurities
(in some cases in conjunction with
transition element impurities), and
the presence of trace elements, which
result in complex crystal lattice
substitutions. The hardness of the
mineral is the same as quartz, thus
it is suitable for use in jewelry.
Hue
and tone
Amethyst
occurs in primary hues
from a light pinkish violet to a deep
purple. Amethyst may exhibit one
or both secondary hues, red and blue.
The ideal grade is called "Deep
Siberian"
and has a primary purple hue of around
75–80%, with 15–20% blue and
(depending on the light source) red
secondary hues. Green quartz is
sometimes
incorrectly called green amethyst, which
is an actual misnomer and not
an acceptable name for the material, the
proper terminology being Prasiolite.
It is actually against FTC Guidelines to
call prasiolite green amethyst.
Other names for green quartz are
vermarine, greened amethyst, or lime
citrine.
Of
very variable intensity, the
color of amethyst is often laid out in
stripes parallel to the final faces
of the crystal. One aspect in the art of
lapidary involves correctly cutting
the stone to place the color in a way
that makes the tone of the finished
gem homogeneous. Often, the fact that
sometimes only a thin surface layer
of violet color is present in the stone
or that the color is not homogeneous
makes for a difficult cutting.
The
color of amethyst has been
demonstrated to result from substitution
by irradiation of trivalent iron
(Fe3+) for silicon in the
structure,[3][5] in the presence of
trace elements
of large ionic radius,[2] and, to a
certain extent, the amethyst color
can naturally result from displacement
of transition elements even if the
iron concentration is low. Natural
amethyst is dichroic in reddish violet
and bluish violet,[3] but when heated,
turns yellow-orange, yellow-brown,
or dark brownish and may resemble
citrine,[6] but loses its dichroism,
unlike genuine citrine. When partially
heated, amethyst can result in ametrine.
Amethyst
can fade in tone if
overexposed to light sources and can be
artificially darkened with adequate
irradiation.
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