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