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Identifying stable opal


#1

Hi, All-

Is there a reliable way to identify an opal as stable? I know that
provenance provides a clue, with Nevada and Australia being on
opposite ends of that particular spectrum, but that is a clue only
and not a guarantee. Unless there is a reliable, objective test for
identifying the stability of an opal, the statements that “stable
opals aren’t fragile” or “stable opals don’t craze” are simply
circular arguments, since the only way to identify a stable opal is
by the fact that it has not (yet) crazed. In the absence of an
objective test for stability, it seems right to notify the customer
of the possibility that the opal they are purchasing may craze. In
any case, it is right to notify the customer that opal is a fragile
stone in comparison with diamonds, sapphires, agates, etc, and that
it does not deal well with – Lee Einer

http://members.cox.net/appealsman/


#2

Unless there is a reliable, objective test for identifying the
stability of an opal, the statements that stable opals aren’t fragile
or stable opals don’t craze are simply circular arguments, since the
only way to identify a stable opal is by the fact that it has not
(yet) crazed What a great statement. It really sum’s up the arguments
on opal. There is no test other than time that I am aware of.

I have been an Opal dealer for the last ten years or so. I have
heard statements from contemporaries to the effect that there are
two types of opal, those that have crazed, and those that will craze.
I know that this is a pessimist view of opal, but depending on the
source of the opal, there might be some truth to it.

As you state, there is no test other than time to know for sure. I
will share my observations though.

  1. The opals from the US are not very stable. Of course there are
    exceptions, but the Nevada Opal will sometimes craze over in a mater
    of minutes. The Idaho opal is also known to be unstable, but when cut
    for triplets, it has a good life expectancy. The Oregon Opal is
    another that may or may not be stable. I have no knowledge of the
    opal from Louisiana, but that is not a major source.

  2. Mexican opals range across the board from very unstable to rock
    solid. Those in matrix seem to be more unstable. I would only
    purchase rough or cut stones that have been dry for a few years. The
    finest opal I ever sold was a Mexican opal faceted into a 7.5 ct
    trillion. It had a water clear body with incredible fire. This stone
    was about three or four years past cutting and I had it under lights
    for over a year before I sold it. It was rock solid. I miss it.

  3. Brazilian opal can also be some of the most stable of opals. Some
    are very hard, and very clear. They are also very hard to find now.

  4. My friends in Australia have a saying that Andamooka is for ever.
    The Andamooka crystal is great. It has a repetition of Never Crazing.
    I have seen stones that were 50 years old that looked new.

  5. Lightening Ridge opals are said to be Craze free. There are some
    fields there that have produced opals that are prone to crazing. It
    depends on the depth of the opal seams. The deeper, the more prone to
    crazing. Some research is needed to find out which field the stone
    came from to determine it’s potential.

  6. Coober Pedy. Some of the fields will produce very sound opal. Some
    will not. The one’s that will that I know of are good are the Dead
    Horse Gully and the Olympic fields. My suggestion here is that if the
    opal has a grey base color, beware of it. The white base and crystal
    seem to be much more stable I will yield my opinions on this to some
    of my Australian friends who have more experience in that area than I
    do.

  7. Other Australian fields are mixed. The Queensland fields of
    boulder opal and Matrix opal are great. The Mintabie fields have
    produced some great stones but again, look out for the grey based.
    Some are prone to crazing while others are rock solid. The White
    based from here is top. The Lambina fields is now producing some
    great opal. It tends to be hard, and some is prone to chipping, but I
    have not seen any crazing as yet. When I say hard, this opal and
    some of the Brazilian opal is running around 7 in hardness. Great for
    faceted stones, but somewhat brittle.

  8. There is opal from Ethiopia, Honduras, and points in Africa that
    are now producing opal. Some are good, some aren’t. I have little
    experience with these locations so I will offer no opinions as to
    their stability.

The bottom line is that you need to know where your opal is coming
from, and you need to do the research to verify the potential for a
long life stone from that area. There is always the exceptions to
what I have stated above, but you should know that you may be taking
a risk. Just remember though, that the greater the risk, the greater
the rewards.

My personal rules for Opal are this;

  1. For rough opal, if it has been dry for two years, and no crazing,
    it should be OK. As I have no idea how you worked the stone, if you
    cut it, it is yours with out any guarantee. If it is uncut, IE the
    same as I sold to you, and there is a problem, we will work it out to
    where we are Both happy.

  2. If it had been cut and under lights for three years, I will
    guarantee it if it has not been set. I do not offer opals for sale
    that have not met the three years criteria.

Opals can be most rewarding. They are the reason I am in the Gem
business. They can also be disappointing. I have a bag of what I call
my heart breaks. They are the ones that didn’t meet my criteria for
sales. IE, they crazed while setting in my show cases. They are
greatly outnumbered by my good stones, but they do exist. I also have
several hundred that in my view are rock solid stones and they make
the Opal business most enjoyable.

Don Rogers