Noel, since synthetic opals are chemically physically and optically
the same as natural opal, it can be difficult to separate them.
Specific gravity, refractive index, fracture, hardness, etc., are
all the same as natural opal. The most prominent giveaway for
synthetic (Gilson) opal is viewed under magnification, and is
usually referred to as the "snakeskin," "scale-like," or "honeycomb"
effect. Under the microscope, synthetics have a typical honeycomb
look within individual patches of color. It also tends to have a
columnar structure when viewed from the side. I believe I have some
good examples to photograph for you if you'd like. I'll bring my
digital camera home tomorrow in case you'd like me to send some.
Another clue is that natural black and white opal may or may not
have a white to moderate blue, green or yellow fluorescence, or none
at all under longwave UV, and it may phosphoresce (glow) for some
time after removing it from the UV light source. Synthetic opal may
or may not fluoresce moderate blue to yellow under longwave UV, but
has no phosphorescence. In a nutshell, quality synthetic opal can
fool even experienced gemologists unless they've examined a good
deal of natural and synthetic opal.
As for imitation, or simulated opal, the separation is usually quite
easy, especially if you've examined a lot of natural opal. Most
imitations are glass, plastic or epoxy based, and magnification
often shows them to have gas bubbles. The imitation glass varieties
bear a superficial resemblance to natural opal. Its RI is generally
1.50 - 1.52, but may vary. The "play-of-color" often resembles
cellophane or tinsel. SG is typically 2.41 - 2.50, but can vary.
Hardness is 5 - 6.
Plastic imitation opal can have an RI of 1.48 or as high as 1.53,
and can also show snakeskin or columnar effect, as well. It often
fluoresces strong chalky bluish-white under longwave UV, and does
not phosphoresce. SG is around 1.20 and can vary. Hardness is a
whopping 2 1/2, so light pressure with a needle will penetrate it.
I'll take some pics of synthetics through the microscope tomorrow
and send off-list if you like, please let me know.
James S. Duncan, G.G.
James in SoFl