Strange opal experience (long explanation)

    After further checking, we find that she purchased it for
about $200.00 on a cruise. We would have estimated the value to be
much higher, based on visual observation. The stone weighs 1.02
carats. It has a dark brown body color, with very good play of
color- large, distinct flashes of red and green." and: "Here is the
weird part: when David got the stone wet during cleaning of the
finished piece, it turned very dark and lost its play of color. We
had the ring soaking in warm ultrasonic solution, though the
ultrasound was not running. The stone looked dark when we dried it
off, but then  returned to its original appearance within
approximately 5 minutes. It also seems softer than regular opal,
with the finish being disturbed even with routine, careful
polishing of the metal around it. We have never seen or heard of
anything like this. Have any of you?" 

Hi Brenda, Yes, I’ve heard of this, and dealt with it a couple of
times, too, but not recently. What I think your customer may have
bought was a cheap piece of Mexican Opal that had been “barnyard
burned” into its present color. Although there are other types of
Opal that’d also yield a brown body color and bright plays of color
(sugar-&-acid _treated Andamooka Matrix Opal, the Koroit Coprolites
and the new Nigerian Opals are but a few cases in point), the
"disappearing fire trick" leads me to believe that the former is the
culprit, and here’s why I think so. You see, back in the mid-to-late
1970’s, an awfully high percentage of the Opal rough produced by the
mines in Queretaro, Mexico, came out of the ground as the usual Fire
Opal – i.e., a translucent to transparent Opal in firelike colors
of orange to red, with or without the spectral play-of-color most
jewelers think of, when they hear of an Opal’s “fire” – but quickly
dehydrated to an unattractive and frequently crazed/cracked,
peach-to-salmon colored potch, called Hydrophane, which is unusable
for jewelry purposes.

In order to really understand this “odd duck” stone you’ve
described, you really need to examine what Opals are about, as a
group. Precious or otherwise, Opals contain trace amounts of water
within their structures, which, in turn, are comprised of
densely-packed, non-crystalline masses of silica spheres, called
"Crystobalite spherules"; in essence, a semi- to mostly-petrified,
hydrated silica gel. Get a piece with a low water content (say, 0.25
to 1%), and with these spherules precisely stacked in a
size-graduated order, and you wind up with a crystal-clear chunk of
Precious Opal with “#5” level play of color (the brightest
possible). Increase that water content to 4-5% and the body of the
stone becomes cloudier, or opaque; jiggle the array of spherules a
few times during their growth process (seismically), and your play
of color dwindles, as well.

In most cases, this water content is fairly low, and the more stable
Opals (like many of those from Mintabie, Australia, and Piaui,
Brazil) bear evidence of this. At the other end of the equation, we
come to Hydrophanes, which have so_much water in them – often as
much as 8-10% – that they ofen will dry out and crack while being
taken out of the ground, or soon thereafter, and are so unstable
that they’re often only available as specimens, packed into a jar
with some sort of colorless fluid, forever to sit atop your favorite
bookcase! (Probably the best examples of this are the partial
replacements of wood, found in the Virgin Valley area of Nevada.
According to some reports I’ve heard, some of these Opalized woods
are still “woody” enough to burn as firewood, once dried.) Anyhow,
Opals from the group, while often among the most beautiful, when at
their best, are usually so “thirsty”, when dry, that they will
actually stick to your tongue as tightly as a leach, if given the
chance. As such, a great many attempts were made to try and
stabilize them, in hopes of realizing silk purses from sow’s ears.

Probably the most easily done of these, and the one I suspect was
responsible for the piece your shop handled, was one which involved
first cutting the stones to their approximate finished size, then
completely drying them out, first in the sun, and later with a
chemical dehydrant. Next, the stones were immersed first immersed in
a solvent-laced then bath of ordinary motor oil, then set to "burn"
in a pile of decomposing horse or cattle dung, where it sat for
several weeks before, finally, receiving its polish. A good number
of similar stones then entered the stone market, and were sold under
names like “South American Black Opals” (origin frequently unknown)
but, because the original, untreated, material was so structurally
unsound, I doubt that many of these “barnyard burned” Mexican Opals
still exist.

The one key detail that “tipped me off” about your customer’s stone
was the fact that it lost its color in the soapy solution in your
ultrasonic tank, even though “the ultrasound was not running”.
Because of the oil fillings in their microscopic pores, these
particular Opals had the “un-Opal-like” tendency to repel water,
rather than absorb it. The only way to overcome this property would
have been to dissolve these carbon-laden oils from the outer
surface, which, in turn, would have left your bench jeweler with a
stone whose outer “skin” was now saturated with a soap, whose
density/opacity, in turn, prohibited the play of color from shining
through. Once dry, all of that “fire” would again transmit from the
stones subsurface (oil-filled) areas, while leaving its newly-porous
outer skin susceptible to “being disturbed even with routine,
careful polishing of the metal around it.” (Hope this makes your day
a happier one.) And so, VOILA! Yet another case solved by Inspector

All my best, Doug Douglas Turet, GJ Lapidary Artist, Designer &
Goldsmith Turet Design P.O. Box 162 Arlington, MA 02476 Tel. (617)
325-5328 eFax (928) 222-0815