Emma Esperon
GEOSC 110H
April 30, 2010
Synthetic Corundum and its Industrial Uses
Corundum is one of the most prized gems in the world subcategorized into
sapphires and rubies, which makes up half of the four major gemstones, accompanied by
diamond and emerald. Both sapphire and ruby have identical chemical structures, Al2O3,
with only slight differences in contaminates, giving them differing colors (Ralph J. and Chau
I. 2010; Ward 1992). Rubies are varying shades of red while sapphires range from inky
midnight blue to a vibrant yellow and all other colors. Corundum also has a high refractive
index of 1.7 and a low birefringence, making it ideal for jewelry. Contributing to its
versatility, Corundum is the second hardest material in the world only preceded by
diamond, making it invaluable in industrial applications. Previous to the creation of
synthetic corundum, sapphires and rubies were used solely for adornment and decoration,
as they were fairly rare and generally small in size. However, availability and crystal sizes
have increased since that time due to the invention of methods to synthesize gemstones.
These processes usually have inclusions that an expert geologist can identify. Used for both
jewelry and commercial uses, synthetic corundum is created, identified, and utilized in
many different manners.
Creation of Synthetic Corundum
Synthetic corundum and natural corundum have the same chemical formula, crystal
structure, appearance, and physical properties. Yet based on inclusions, a well-trained gem
appraiser can generally tell the difference between the pieces of corundum grown in a lab
Esperon 2
from the gem formed deep under the earth. Marring the inside of the gemstone, some
types of flaws can only be found in synthetic or natural gems, while many stones possess no
flaws at all. However, some of the specks and flecks that arise in synthetic gems are due to
and unique to the process that created it.
Creating Synthetics
The most common process to synthesize corundum is
the Verneuil Process, which was created by August Verneuil
at the turn of the century. This process, also called Flame
Fusion, sifts powdered aluminum oxide combined with a
coloring agent through a sieve when a mechanical hammer
taps on the side of the raw material tank (Encyclopædia
Britannica 2010; Read 2005; Post 1997; Gemstone Buzz
2009). Then the small particles of alumina are forced down
Read 2005
a superheated pipe filled with oxygen. The particles are liquefied in the heart of an
oxyhydrogen flame at around 2,030°C, corundum’s melting point. The melted gem material
then drips onto a seed crystal and forms a boule, a rough cylindrical crystal. The boule
grows to the desired size, usually twelve inches long and about 200 to 500 carats. This is
quite a feat since natural gem quality rubies are very rare over ten carats. In a little over
four hours the boule is removed from the contraption and cooled, solidifying the rough
synthetic gem.
The Verneuil Process requires precise timing. The surface area of the stone cools
more quickly than the interior, putting stress on the gemstone (Encyclopædia Britannica
2010; Read 2005; Ward 1992). In order to avoid serious cracks that could ruin the entire
Esperon 3
gem, a gem maker must split the stone vertically, relieving tension. To avoid this part of
the process entirely, some synthetic gem growers slowly cool their boules in a furnace or
repeat the heating and cooling cycle, making the corundum less brittle. In this fashion any
size or color corundum crystal can be grown, just by varying the consistency of the powder
melted. In order to produce a ruby, the gem must contain the coloring agent, chromium
oxide. A blue sapphire requires titanium, cobalt yields a green hue, and yellow is achieved
through nickel and magnesium oxides. The multiple colors and sizes of corundum offered
by Flame Fusion is a relatively quick and simple process yielding over 100 million carats or
corundum yearly. The available large quantities also devalue the gem as Verneuil
corundum is worth less than a dollar per carat. Due to their inexpensiveness and other
physical properties this synthetic process creates wonderful gems for industrial and
commercial use, while other processes are more specialized.
Specifically used to synthesize gem quality stones and discovered in the 1980’s, the
Flux-Melt process is a solvent-based technique using aluminum oxides and coloring agents
suspended in a platinum crucible (Read 2005; Ward 1992). Silica glass combines with a
supersaturated solution, flux, which surrounds the alumina. Sitting at 800 °C, the source
material in the solution dissolves in the flux at the
bottom of the crucible, where it is closer to the
heater and warmer. Then the flux rises to where
the seed crystals are suspended and cools just a few
degrees allowing the source material to precipitate
and grow onto the gem. Since this process takes
two to ten months for the flux to form moderate
(Read 2005)
Esperon 4
sized gems, source material is the limiting agent and must be periodically added so that the
crystals can continue to enlarge. When the growing time is up any extra flux is drained out
of the heat resistant platinum crucible and the corundum gems are separated. This process
creates more valuable gems than the Verneuil Process, as the gems sell for several hundred
dollars per carat. However, to achieve the perfection required in synthetic corundum for
industrial use a different process is often implemented.
One last technique that is used to create synthetic rubies and sapphires is the
Czochralski Method also called the “crystal pulling” process. Designed in 1918, this process
involves a liquid melt of alumina heated in a crucible by a radio-frequency induction coil
(Read 2005; Ward 1992). Then the source crystal
is lowered into the melt on a pull rod until the seed
just kisses the surface of the molten gem material.
Slowly, the seed stone is rotated and raised at a
steady rate allowing the source material to attach
and grow new layers in a downward direction.
This process produces incredible clear and pure
corundum, but there are some hazards hidden
within the procedure. The temperature of the
(Read 2005)
melted gem is critical, as it must hover a few degrees above the melting point of corundum.
If the temperature goes too high then the seed crystal will melt and if the temperature goes
to low then the seed stone will bind with the source material; either scenario would
destroy the potential synthetic gem as well as the seed crystal. This process can be applied
Esperon 5
for many stones but since the results are virtually free of imperfections they are highly
valued in the industrial markets.
Identifying Synthetics
Although synthetic gems have the same chemical properties as natural gems, there
are sometimes clues, inclusions, allowing gem experts to determine the authenticity of the
stone. Gemstone experts originally identified Verneuil corundum by gas bubbles and
growth striations; however, by slowing the growth cycle and increasing the hydrogen and
carbon content these problems have
become less noticeable (O’Donoghue
2006; Hughes R.H. 2010; Golecha March
2008). Furthermore in the corundum
trade, unlike the tourmaline business, if
a sapphire transitions color, for
example one end is blue and the other is
red, it is clearly a synthetic gemstone.
The Verneuil gems also suffer from
(Read 2005)
cracks and stress that can best be
solved by keeping the boule smaller in size. At times the color and patterns within the gem
reveal its true identity. For example, curved color banding is visible when viewing a
sapphire through a blue filter on a microscope. However, synthetic pink, yellow, orange,
and brown sapphire all form practically without inclusions, but these stones can sometimes
be identified due to their florescent abilities. These color fancy sapphires, with the
exception of brown, phosphoresce after exposure to X-rays, while under normal conditions
Esperon 6
natural sapphire stones would not phosphoresce due to their increased iron content.
When exposed under shortwave ultraviolet luminescence (SWUV), synthetic pink
sapphires are transparent radiating a violet color, while natural pink corundum is opaque
glowing crimson. However, detecting synthetic sapphires by SWUV tests are sometimes
inconclusive because it can only detect Verneuil sapphires. Stones created by other
processes contain too much iron and appear like a natural gem, causing scientists to
explore other telltale inclusions associated with the alternative processes.
The Flux-Melt method is not without flaws yet these gems often appear the most
natural until they are put under close scrutiny (Ward 1992; O’Donoghue 2006; GeoGem
International Jewelers 2010). The flux often gets trapped within the stone creating small
bubbles of liquid that solidifies after the surrounding gem material, creating inclusions
called fluid cavities. The flux method also mars the stone with fingerprint inclusions that
appear like liquid feathers or veils. Also sometimes small flecks of the crucible, called
platinum platelets, merge with the gem material causing a telltale inclusion. The presence
of unnatural inclusions as well as the absence of the natural ones can indicate that a
gemstone was synthetically created.
While the Flame Fusion process and Flux-Melt technique can abound in
imperfections, the Czochralski Method creates practically flawless corundum boule
(Gemstone Buzz 2009). In this case the lack of natural inclusions is one of the few ways to
identify this style of synthetic sapphire and ruby. With only the rare gas bubble or curved
line to mar its beauty, this style of gem is the most valued synthetic corundum. It is often
used in industrial uses where any imperfections could hinder the application of the gem.
Esperon 7
Industrial Uses of Corundum
The creation of pure synthetic corundum has expanded the uses of sapphires and
rubies to create components of commercial products. A number of corundum’s physical
properties have diversified its usages from merely that of adornment purposes. An
incredible hardness and a distinctive fluorescent property distinguish this gemstone from
others as it is implemented to create and comprise common machines and tools.
Hard and Abrasive Corundum
One of Corundum’s properties that make it desirable is its incredible hardness,
ranking a nine on Mohs Hardness Scale (Ward 1992). This characteristic is vital to the
watch-making business. Clear synthetic sapphire is cut to form the faceplate in the watch
preventing the screen from being scratched or scared by physical or chemical abrasives.
Ruby is also sliced into fine circular bearings to be used in the gears of watches. These
hardy crystals reduce wear and friction within the watches, allowing the timepieces to last
longer. In fact, 200 million carats of synthetic corundum is yearly used to create watches.
Sold for over four hundred dollars, a Swiss watch keeps the time with the help of durable
synthetic corundum crystals.
The extreme hardness also makes corundum am excellent material to make
protective panels in other commercial objects as well (Ward 1992). A common item that
has a large clear sapphire faceplate is the laser scanner at a grocery checkout lane. A thin
layer of clear sapphire, just 1/32 of an inch protects the underlying glass laser window,
lasting for about ten years. So sapphire remains strong in situations where glass or quartz
would get scratched by the daily flow of cans, bottles, and other goods. This means that the
laser light ray can accurately identify barcodes without irregular refractions due to
Esperon 8
scratches in the plate. The sapphire acts as an effective protective coding due to its tightly
compacted bonds.
The close-knit atoms also make it an extremely good abrasive (2010 Corundum;
Ward 1992; SaveYourSmile Inc.). When corundum is crushed into fine particles it can be
used as a key component in sandpaper, grinding wheels, cutting machines and personal
hygiene products. In fact, nail files are coated with emery, which is mainly comprised of
granular corundum. Corundum also appears in toothpaste in the form of “alumina.” This
can be one of the abrasives in toothpaste, which acts to polish teeth, removing plaque and
stains. The same synthetic corundum base is contained in many more commercial
products, contributing to the rough tool that serves to polish so many diverse surfaces.
Ruby’s Fluoresce
Another physical property of corundum is its florescent ability, which makes it
valuable to the industrial business as well as the jewelry trade (Taylor 2005). Natural
ruby’s ability to fluoresce is sometimes diminished by its higher iron content, which makes
a pure synthetic ruby created by the Czochralski Method most desirable. Naturally
absorbing shortwave ultraviolet light (SWUV), ruby crystals convert and transmit the
energy into wavelengths that fall into the red
spectrum. This is because chromium within the
crystals undergoes electronic shifts in energy. The UV
light within the natural sunlight can trigger this effect
causing rubies to slightly fluoresce and appear a more
dazzling red color. Synthetic rubies excel at
emanating long red wavelengths of visible light when
(Russell 2010)
Esperon 9
given the correct stimulus in a commercial tool.
The laser pointer, created in 1960, is one of the tools that manipulate ruby’s ability
to fluoresce (Taylor 2005; Weschler 2010). In order for this to function correctly, the ruby
must be a pure synthetic that can only be created by the crystal pulling method. Any
imperfections in the gem could quench the fluorescing ability or cause unwanted refraction
when the light beam is concentrated. Along with the ruby in the laser there is a flash tube,
which is much like a flash bulb from ancient cameras. This flash tube gives off UV light that
is absorbed by the ruby and excites the ruby’s
atoms causing the atoms to emit photons. The
light rays are then focused by mirrors and shot
out one end of the laser to create a visible red
point. The practicality of lasers is immense and
is used in a large number of common items. CD
(Weschler 2010)
players contain ruby lasers in order to read divots on disc, containing information. Microsurgeons use ruby lasers to burn away bodily tissues while industrial facilities use highenergy ruby lasers to cut through steel and drill holes in the ruby bearings used in watches.
All of these many uses are facilitated by the ruby’s innate ability to fluoresce, which is
harnessed into a useful apparatus.
Conclusion
Making corundum a coveted possession for jewelry dealers and the industrial
business, the plethora of commercial applications for rubies and sapphires abound. With
technological breakthroughs in the synthesizing processes, sapphires and rubies can be
created to satisfy the increasing commercial demand. However, the three main methods of
Esperon 10
synthesizing corundum can leave inclusions within the gemstones. Devaluing the gem,
these flaws can indicate what technique was used to create the gem and limit the purposes
that the boule can be used for. The synthetic methods of creating corundum have allowed
corundum’s physical properties to be manipulated and applied to industrial use,
permeating society.
Esperon 11
References Cited
Ralph J. and Chau I. (2010) Corundum, Mindat, <http://www.mindat.org/min-1136.html>.
Encyclopædia Britannica (2010) Verneuil process, Encyclopædia Britannica Online,
<http://www.britannica.com/EBchecked/topic/626323/Verneuil-process>.
Gemestone Buzz (2009) Corundum, <http://www.gemstonebuzz.com/corundum>.
GeoGem International Jewelers (2010) Synthetic Gemstones, BBB Accredited Business,
<http://www.geogem.com/syntheticgem.html>.
Golecha, C. (March 2008) A Crude Method to Test SWUV transparency: A helpful test for
yellow-colourless-light coloured sapphires, Lab Information Circular,
<http://www.golecha.blogspot.com/>.
Hughes R.H. (2010) Ruby & Sapphire, Chapter 5: Inclusions, RWH Publishing & Books
<http://www.ruby-sapphire.com/r-s-bk-ch5.htm>.
O’Donoghue, M. (2006) Gems, 6th Edition, 481-493 p. Elsevier Ltd, Oxford.
Post, J. (1997) The National Gem Collection: Smithsonian Institution, Harry N. Abrams, Inc.,
New York.
Read, P. G. (2005) Gemology, 3rd Edition, 168-174 p. Elsevier Ltd, Oxford.
SaveYourSmile Inc. (2010) Everything You Wanted to Know About Toothpaste: Toothpaste What's In It?, Philadelphia, < http://www.saveyoursmile.com/toothpaste/
toothpaste-c.html>.
Russell, R. M. (2010) Windows to the Universe, National Earth Science Teachers
Association, <http://www.windows2universe.org/physical_science/
magnetism/images/visible_spectrum_waves_big_jpg_image.html>.
Esperon 12
Taylor, A. (2005) Gemstones and their Industrial Uses, Field Geology Club of SA Bulletin,
Vol. 34, No.7, 7-9 p. <http://www.fgcsa.bigpondhosting.com/GemstonesInd.htm>.
Ward, F. (1992) Rubies & Sapphires, Gem Book Publishers, Bethesda.
Weschler, M. (2010) How Lasers Work: Ruby Lasers, HowStuffWorks, Inc.,
<http://science.howstuffworks.com/laser5.htm>.
(2010) Corundum: Mineral Uses & Properties, Geology, <http://geology.com/minerals/
corundum.shtml>.