Gemstone variety of chalcedony
Chrysoprase
,
chrysophrase
or
chrysoprasus
is a
gemstone
variety of
chalcedony
(a
cryptocrystalline
form of
silica
) that contains small quantities of
nickel
. Its color is normally apple-green, but varies from
turquoise
-like cyan to deep green. The darker varieties of chrysoprase are also referred to as
prase
. (However, the term prase is also used to describe
chlorite
-included quartz, and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.)
Chrysoprase is cryptocrystalline, which means that it is composed of crystals so fine that they cannot be seen as distinct particles under normal magnification. This sets it apart from rock crystal,
amethyst
,
citrine
, and the other varieties of crystalline quartz. Other members of the cryptocrystalline silica family include
agate
,
carnelian
, and
onyx
. Unlike many non-transparent silica minerals, it is the color of chrysoprase, rather than any pattern of markings, that makes it desirable. The word chrysoprase comes from the
Greek
χρυσ??
chrysos
meaning 'gold' and
πρ?σινον
prasinon
, meaning 'green'.
Unlike
emerald
which owes its green color to the presence of
chromium
, the color of chrysoprase is due to trace amounts of
nickel
compounds in the form of very small inclusions. The nickel reportedly occurs as different silicates, like
kerolite
or
pimelite
(not NiO mineral,
bunsenite
, as was reported before). Chrysoprase results from the deep weathering or
lateritization
of nickeliferous
serpentinites
or other
ultramafic
ophiolite
rocks. In the Australian deposits, chrysoprase occurs as veins and nodules with brown
goethite
and other
iron oxides
in the
magnesite
-rich
saprolite
below an iron and silica cap.
As with all forms of
chalcedony
, chrysoprase has a hardness of 6 to 7 on the
Mohs hardness scale
and a conchoidal fracture like
flint
.
The best known sources of chrysoprase are
Indonesia
,
Queensland
,
Western Australia
, Haneti
Tanzania
,
Germany
,
Poland
,
Russia
,
Arizona
,
California
, and
Brazil
. Deposits in central
Tanzania
have been in constant production since 1986. The chrysoprase and nickel silicate ore deposit in
Szklary
,
Lower Silesia
,
Poland
, was probably the biggest European chrysoprase occurrence and possibly also the biggest in the world.
[
citation needed
]
A very similar mineral to chrysoprase is
chrome chalcedony
, in which the color is provided by
chromium
rather than nickel.
See also
[
edit
]
References
[
edit
]
External links
[
edit
]
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Gemmological classifications by E. Ya. Kievlenko (1980), updated
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Jewelry
stones
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Jewelry-Industrial
stones
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Industrial stones
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Related
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