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Gem ID for "Peruvian Opal"


#1

Over the past few years I have purchased a few stones from various
dealers that they have called “Peruvian Opal.” It is a sea green
stone that looks very similar to chalcedony, except for the color.
So I wonder if, in fact, it is dyed chalcedony. If any of you have
some insight on this, I’d really appreciate knowing the proper gem
identification.

Thanks,
Bonnie Cooper


#2

Bonnie,

By what means can a mineral identification be made when all that is
given is a brief description of color? You should be able to test
the hardness and general specific gravity easily enough, but, if you
cannot, take it to someone with some mineralogical/gemological
training. The two materials do not look like each other, and you
probably have opal; hardly worth the effort of dying small pieces of
chalcedony. Chalcedony is usually dyed AFTER shaping and polishing.
I have never, ever seen a dyed iregular chunk of chalcedony. It’s
probably opal, and the luster should be vitreous to waxy. The piece
should feel “light” in the hand; but none of this takes the place of
examination with a microscope and proper tools by a trained person.

Your description is simply not sufficient to make a proper
identification; it’s sort of like saying “Hey, I have some yellow
metal here, is it gold?”

Wayne Emery
The Gemcutter


#3

Hello Bonnie;

Peruvian Opal is bluish in color. Your description fits the apple
green opal from Macedonia,

http://www.creativegem.com/gem_deals/gem_db1/s8656.jpg

Someone is to see the stone in order to offer you a Gem ID

Mark


#4

I can only guess not having the stone to look at; but there is PBO
(Peruvian Blue Opal) as it’s commonly called. It doesn’t look like
opal one usually sees; but is mineralogically speaking opal. Only a
guess as a ‘pig in a poke’.

KPK


#5

There is a blue (green?) common opal that comes out of Peru. Opal
can be almost any color of the rainbow. There is some neat strawberry
colored stuff.

Rose Alene


#6

Bonnie

Your question is difficult to answer for several reasons.

  1. Opal, chalcedony and quartz are all the same thing chemically,
    they only vary in how they are produced.

  2. Not all Opal is, like the precious opal, there are clear (Mexican
    Fire Opal) and colored varieties which may or may not have the color
    flashes we expect. There is also material called common opal (potch)
    which abounds, and currently from the advertising, I believe 4 from
    Peru, Pink, Blue, Green, White which are available as rough.

  3. Cutting the stone would probably be the only way you could tell if
    it had been dyed assuming that the process had not been performed
    after shaping.

As I don’t have your stone, I can not tell. Chalcedony and Opal are
both porous and both would be susceptible to coloring. Most
chalcedony and Potch Opal are stable, but there are exceptions.

Sorry I could not provide a better answer, but maybe one of the GIA
folks would care to chime in. I rely on obtaining rough from sources
with the matrix still attached, quality dealers who could ill afford
the scandal, or to personal collecting. (My favorite since I was
about 4 years old)

Sorry I could not provide you a better answer, but it seemed the
genealogy of opal and chalcedony might be in order on this question.

Terry


#7

Bonnie-

There really is a true Peruvial opal out there, but it is typically
more of a sky blue rather than green in color. At least, I have never
seen any color but various shades of blue. (I’m not sure, but the
blue might be a result of trace amounts of copper ions in the opal.)
Like anythiing else, it is also possible that something offered by a
dealer might be a material other than opal, and could of course be
dyed to imitate the real thing. I have only looked at rough material,
not finished stone. The opal I have inspected has been milky, and
could be described as somewhat like chalcedony in general appearance.
Opal is much softer than chalcedony, so a hardness test is a pretty
good diagnostic.

Dick Davies
Lapidary


#8

Bonnie-

While we don’t know that your stones are actually “Peruvian Opal”,
there is such a thing. The material has been on the market for many
years and is a variety of common opal from Peru. In addition to blue
to green material, there is also pink common opal from Peru.

Opal is a hydrated silicon dioxide with no crystal structure. Common
means the opal has no play of color and is generally more opaque
than “precious opal”. Because of it’s water content, this opal can
dry out and craze.

Chalcedony has the same basic chemistry, but is a variety of quartz,
having a crystal structure albeit on the microscopic level. Opal has
no crystal structure, which is part of the reason it is softer than
quartz.

A refractive index can be a reliable way to separate opal from
chalcedony, if you have a good polished surface + the tools and
skills required. Opal will read ~ 1.45, while chalcedony would be
~1.54. Specific gravity can be another way to identify opal, but not
as straightforward, because of the presence of matrix materials.
Hardness tests are only useful if you are dealing with rough
material and have experience as a lapidary-- destructive tests such
as hardness are not used in most gemological id’s.

There is plenty of fake or doctored “Peruvian Opal” (mostly dyed
stuff) floating around out there, so be sure you buy from a
knowledgeable source you can count on. Some of this dyed material is
very good. Dyed material may have color concentrations around cracks
and the characteristic dendritic formations you see in this type of
opal. Other dye jobs may show with a simple swab test using acetone
or alcohol, however much of the dyed material is very difficult to
detect and must be sent to a top gem lab for identification. I spoke
to one of the top opal cutters/experts in the country and he said he
often cannot tell, and must rely on labe. My advice is don’t try to
buy based on what you think you know-- rely on your source.

Jim Sweaney, CGA, GG, FGA


#9

Bonnie; The Peruvian opal in question is indeed the real thing. As
far as I know it is not treated in any way, unlike the chalcedony
which is died. I have seen pieces of rough that look like frozen
Carribean Ocean but there are lower grades also usually with brown
streaks. Another interesting feature of this opal is the black
dendrites which can occur in the stone.

Dave Owen


#10

I would just like to mention that this opal can be “cracky”; I
bought several cabs, and found that after 6 months or a year, cracks
often began to appear.

Margaret