Agates and Opals
|
|
-
finely crystalline, microcrystalline or cryptocrystalline quartz
-
termed chalcedony:
often fibrous, botryoidal masses, fibres perpendicular to hummocks
-
S.G. ~ 2.6 (lower than quartz, slightly porus)
-
silica rich solutions fill up cavities in rocks to form
geodes
-
porosity allows coloration by dyes:
-
Agate
is most common variety of chalcedony
-
crystals
at core may be amethyst note the color bands
|
|
-
it's made from essentially amorphous or non-crystalline material
-
despite the lack of crystal structure, opal does display internal order!
-
composition = SiO2*nH2O (up to 30 % water)
-
hardnesss = 5.5 -6.0
-
color = variable, with play of color
-
"opalescence" = color change with orientation
|
View
some opal images!
|
Opal Varieties
There are three general opal groups:
Precious Opal
Precious opal, which displays 'opalescence' (spectral color,
irridescence that changes with the angle at which the gem is viewed).
Two subgroups:
White opal is an opaque stone in which the colors appear as flashes
or speckles.
Black opal contains fire with a dark body color. These are less
common and tend to be costly.
Examples:
precious
opal note the rainbow-like iridescence.
colors
and patterns
opalized
wood
shell
stem
|
|
Fire Opal
Fire opal is transparent or translucent with an orange or red
body color. Fire opals are named for their color (but are not opalescent).
The term is often misinterpreted to indicate that fire opal is opalescent
because in precious opal, (with a play of color) the play itself is called
fire.
Examples of fire opal and related varietites
(Mexican) water opal is transparent and contains flashes of fire.
jelly
opal
also found in a rhyolite
matrix
"water-bearing
opal"
Water can still be seen inside the opal!
fire
opal
note
the orange color, no opalescence
|
|
Common Opal
Common opal is rather opaque. Many names are used to describe
varieties, e.g., honey opal, milk opal, moss opal, etc.
Examples:
common
opal: opaque, no play of color
"petrified" opal
|
|
Where is opal formed and where is it found?
Geological:
precipitated near the Earth's surface from
circulating ground water or hydrothermal solutions; esp. where seasonal
rainfall and extended dry periods, often as linings of cavities or crack
fillings
Geographical
90 % of World's supply of opal from South Australia
|
|
Assembled Opal Products
Doublets and Triplets:
Doublets and triplets consist of a thin slice of opal cemented
to a dark substrate. In the case of opal triplets, a colorless glass or
crystalline cap is cemented to the opal slice.
Doublets and Triplets are common, as these are substantially less expensive
than precious opal.
Examples:
doublets
triplets
|
|
Enhancements
-
impregnation with plastic, surface oiling
-
carbon or sugar treatment:
-
heating the opal in paper (manure) leaves a deposit of carbon below the
surface of the stone
-
soaking the opal in sugar-rich solutions in sulfuric acid bath gives it
a peppery
'pinfire' effect (play of color)
(this is a superficial treatment, which leads to low S.G. and porus
stones)
|
Preservation
and handing of opals |
Synthetic Opals
There are several manufacturers of synthetic opals, including Gilson
Opal, Inamori Opal, etc.
Simulants
|
|