Spinel - The Misunderstood Stone
by Bill Vance G.G.
I believe one of the most misunderstood gem stones in the world is spinel. Mention spinel
and most people automatically think synthetic or man made. Man made spinel was
produced by accident around 1920. The verneuil process of creating sapphire was being
experimented with to produce a more realistic looking sapphire, someone added
magnesium and viola - a new synthetic was born. The verneuil man made (VMM) spinel
has a slightly higher refractive index (RI) approximately 1.73 and shows anomalous
double refraction (ADR). The VMM spinels are produced in a vast number of colors,
including colorless, but it most often mimics aquamarine. Natural aquamarine & VMM
spinels are easily separated using an aqua or chelsea filter, the VMM spinel is colored by
cobalt and will look pink or red through the filters. Fluorescence will also assist in
separating the VMM spinel from the natural. Any time a 1.73 RI single refractive stone,
other than red, shows fluorescence, the stone is man made. Some natural red spinels from
Africa will show a red fluorescence. With the exception of the red, all natural spinels are
inert, which means they don't react to short wave or long wave ultra violet light. Red
spinels seem to have always played havoc with mans effort to put a name on something.
In the old days the name carbuncle, which means "small coal or ember", was used for
most red stones. Garnet, ruby and spinel were all, at one time, classified as carbuncle.
When hardness was discovered to be an aide in identification, garnet at 7-71/2 hardness
was removed from the list. But the difficulty of separating spinel, 8 in hardness, from
ruby, 9 in hardness, still existed. Some of the largest and most famous rubies in the world
are really spinels not corundum. The most classic example of this confusion is the "Black
Prince's Ruby" in the English Imperial State Crown, it is not a ruby but a spinel. This
startling discovery was made about 120 years ago, when it was learned that some stones
were single and some stones were double refractive. Refractive index (RI) is measured
when a ray of light leaves one medium and enters another, such as a stone. The
proportional relation between the speed of light in air to that of the stone, is the the
refractive index (RI) and varies in a tell tale way from stone to stone. Natural spinels RI
of 1.72 and single refraction will easily distinguish them from ruby with its RI of 1.76-
1.77 and double refraction. All of this wonderful technology is pertinent to distinguish
spinel from other stones. But now there is a new flux grown man made spinel that so
closely mimics the natural that only a professional with a well trained eye, accustomed to
natural versus flux inclusions, can separate them. As time goes on perhaps more colors
than the current red and blue will be produced. But for now these colors provide a real
challenge, even to the trained eye.
Natural or noble spinel exists in a wide range of colors. The favorite being ruby red but it
is also found in pink, orange, blue, purple, yellow, dark green, black and even blue to
purple color range. Spinel is found in fewer locations than corundum (ruby & sapphire)
and sometimes is found with it. Spinel, in my opinion, will undoubtedly increase in value
as this wonderful gem material becomes more appreciated by collectors and the general
public.
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