Glass and other simulants go back a LOT farther than the 60’s.
While glass has been widely used as a simulant, it’s not the only
one. Glass could concievably have the color change you describe,
but it closely resembles some synthetic spinel I recall seeing
way back when, as well as some of the more common, synthetic
sapphire type of simulant. The latter is very common. This is
not “lab” alexandrite, since it is not chemically the same as
alexandrite. It’s color change goes from purplish/amethyst
color to a blue/grey color. The spinel version has a stronger
blue, and the purplish color under incandescent is redder, though
Testing any simulant is sometimes easy, sometimes not. If you
can find spherical high relief gas bubbles in the material, it’s
usually glass or plastic. No natural gems have round bubbles of
gas in them. but glass manufacturers have gotton pretty good
and preventing these. And very old flame fusion types of
synthetic corundum, most commonly ruby, also sometimes have
spherical gas bubbles. You need a good loupe to see these
clearly. A penlight lighting the stone from the side or back
also helps a lot.
Glass also sometimes shows distinct "flow lines. These are
simply the syrupy curvy lines of flowing glass as it’s poured,
quite unlike the usually geometric or straight zones of color you
can find in real gems. Think of the appearance of honey poured
into water and not completely stirred. visible swirly streaks.
Glass and plastic and amber are the main things that can show
this. Again, easier to see with a loupe.
A definative identification usually needs examination with both
a polariscope and refractometer. The latter is expensive, but a
polariscope is just two pieces of polaroid held at 90 degrees to
each other. In that orientation, light coming through both will
be blocked, and you’ll see what looks like dark purple only, when
you look at a light through crossed polaroids (Like the lenses
from a scrap pair of cheap sunglasses, for example. You need only
rig up some way to hold then at 90 degree rotation to each
other, one above the other, with a space between in which you
hold your gem) Anyway, crystaline materials held between those
two polaroid lenses show interesting effects. Cubic system
materials, including spinel, garnets, and diamond, usually stay
dark looking, though often streaky. Glass gets a cross shaped
bright pattern that stays oriented to the polaroids, no matter
how you turn the gem. Most other gems, like alex, sapphire,
etc, as you turn them, they blink from light to dark. The
appearance of glass under a polariscope is fairly distinctive,
and easy to do. You do have to buy or make a polariscope
though. Commercially made nice units cost several hundred. But
like I said, you can put a couple saw slots in a scrap of wood,
and use those to hold a couple lenses from a pair of cheap
polaroid sunglasses, and though it’s a bit crude, it will work.
Of course, your best bet would be to simply take the stone in
question to someone with the ability to identify it. Most
jewelers, for example. Might cost you a few dollars for someone
to see if it’s glass, but that’s still easier than going to some
great effort to make a polariscope and learning to use it…
Hope this helps.
Peter Rowe G.G.