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
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