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Distinguishing synthetic from natural opal?

Hello all,

I’ve been ebaying again…well, actually, this was bought about the
same time the others were, but has only just arrived. I bought a
large, bright opal that the seller (australian woman) had inherited
in a parcel her parents had bought in the 1970’s. She was told it
MIGHT be synthetic, so never did anything with it. So she put up the
last remnants of that parcel on ebay and I bought the largest of the
stones, a 23x18 oval, red green crystal base (some blue/purple in
bright light), nice bright stone…color everywhere! 10.7 cts. There
is so much fire that the only way you can see it is crystal is to
hold it up w/ a light behind it…then you can see through it.

Anyway, I know that it’s worth a lot more than the $12 I paid for it
w/ postage no matter what it is…but how can I tell if it is natural,
or for example, a Gilson? I can see it has been manually cut because
the edges are not polished as well as the top and the back is not
completely finished. It is a bright stone, even in subdued roomlight.


Hi Jeanne. If the seller’s parents actually bought the parcel in
question in the 70’s it is certainly possible that your opal is a
synthetic, and possibly Gilson, since Pierre Gilson invented it in
1974. Still, it wasn’t very widespread at that time, so it is also
possible that it is natural.

Gilson has done such a good job in creating their opal that it is
difficult for the untrained eye to separate it from natural opal.
There are, however, some differences that a trained eye can spot.
The most prominent giveaway is something known as the “snakeskin
effect” that is best seen with a microscope, but may also be
detected with a loupe. Look closely at a patch of color and try to
determine if it has a scale-like appearance. It’s easy to miss if
you don’t know what to look for. Also, the play-of-color of most
synthetic opal has a columnar structure when viewed from the side,
depending on how it was oriented when cut. As a self-avowed
opalholic, I’ve looked at a lot of them under magnification, and I
have seen both “snakeskin effect” and columnar structure in natural
opal, as well. It can be difficult to tell the difference sometimes
between natural and synthetic.

Another thing is fluorescence and phosphorescence. If you have
access to longwave and shortwave UV light, darken the room and check
the stone with them. If it (a synthetic) has a white base color, it
may show moderate blue to yellow fluorescence and no phosphorescence
(glows a bit after you turn off the light) under longwave. Under
shortwave, it may show moderate to strong blue to yellow
fluorescence and weak phosphorescence.

If a black base color, it could show anywhere from no reaction to
moderate yellow with no phosphorescence under LW, and none to weak
yellow under SW. If it is a true crystal base as you say, it
probably isn’t synthetic, but many Gilsons appear to be crystal, but
aren’t. Also keep in mind that batches of synthetic opals vary in
character quite a lot, and all of the above may or may not apply to
yours. It takes some experience to separate certain synthetics from

There are also minor differences in refractive index and specific
gravity, but they overlap and can be confusing and I’m guessing that
if you had a refractometer, you wouldn’t be asking the group.

I am always loath to ID a gemstone from a photo, but some Gilson
does have a telltale structure. If you have a pic, let me know
off-list. But don’t count on it.

James in SoFl


Check the stone under magnification with good lighting. A microscope
with dark field lighting works great.

On the synthetic Opals you will see a honeycomb effect under

Good Luck
Greg DeMark
Longmont, Colorado
Custom Jewelry - Handmade Jewelry - Antique Jewelry

I do have gemological equipment, but all of it is packed and hard to
get at because I’m in the process of moving to the USA. I cannot see
the scale like pattern, but it would help to have an example of that
to know for sure what I’m looking for. Will see if I can find that on
the net. For anyone interested, I’ve put up a page w/ this stone and
the others I mentioned in another email at

The picture of this opal I too yesterday in direct sunlight w/ a
supermacro lens. The pictures of the other opal were taken from the
ebay auctions themselves.


For anyone interested, I've put up a page w/ this stone...

As usual, it’s impossible to tell for certain from a photo. While
the opal in question looks characteristically like a white base
synthetic such as Gilson, it could as well be natural. But as you
mentioned to me off-list, it is certainly worth more than $12 US.

James in SoFl