Ruby
A
ruby is a pink to blood-red colored
gemstone, a variety of the
mineral corundum (
aluminium oxide). The red color is caused mainly by the presence of the element
chromium. Its name comes from
ruber,
Latin for red. Other varieties of gem-quality corundum are called
sapphires. The ruby is considered one of the four
precious stones, together with the
sapphire, the
emerald and the
diamond.
[1]
Prices of rubies are primarily determined by color. The brightest and
most valuable "red" called blood-red, commands a large premium over
other rubies of similar quality. After color follows clarity: similar to
diamonds, a clear stone will command a premium, but a ruby without any
needle-like
rutile inclusions may indicate that the stone has been treated. Cut and
carat
(weight) are also an important factor in determining the price. Ruby is
the traditional birthstone for July and is always lighter red or
pink than
garnet.
Physical properties
Crystal structure of ruby
Rubies have a
hardness of 9.0 on the
Mohs scale of mineral hardness. Among the natural gems only
moissanite and
diamond
are harder, with diamond having a Mohs hardness of 10.0 and moissonite
falling somewhere in between corundum (ruby) and diamond in hardness.
Ruby is α-alumina (the most stable form of Al
2O
3) in which a small fraction of the aluminium
3+ ions are replaced by chromium
3+ ions. Each Cr
3+ is surrounded octahedrally by six O
2− ions. This crystallographic arrangement strongly affects each Cr
3+,
resulting in light absorption in the yellow-green region of the
spectrum and thus in the red color of the gem. When yellow-green light
is absorbed by Cr
3+, it is re-emitted as red
luminescence.
[2]
This red emission adds to the red color perceived by the subtraction of
green and violet light from white light, and adds luster to the gem's
appearance. When the optical arrangement is such that the emission is
stimulated by 694-nanometer photons reflecting back and forth between
two mirrors, the emission grows strongly in intensity. This effect was
used by
Theodore Maiman in 1960 to make the first successful
laser, based on ruby.
All natural rubies have imperfections in them, including color impurities and inclusions of
rutile
needles known as "silk". Gemologists use these needle inclusions found
in natural rubies to distinguish them from synthetics, simulants, or
substitutes. Usually the rough stone is heated before cutting. Almost
all rubies today are treated in some form, with heat treatment being the
most common practice. However, rubies that are completely untreated but
still of excellent quality command a large premium.
Some rubies show a three-point or six-point
asterism or "star". These rubies are cut into
cabochons
to display the effect properly. Asterisms are best visible with a
single-light source, and move across the stone as the light moves or the
stone is rotated. Such effects occur when light is reflected off the
"silk" (the structurally oriented
rutile
needle inclusions) in a certain way. This is one example where
inclusions increase the value of a gemstone. Furthermore, rubies can
show color changes—though this occurs very rarely—as well as
chatoyancy or the "cat's eye" effect.
Color
Generally, gemstone-quality corundum in all shades of red, including pink, are called rubies.
[3][4]
However, in the United States, a minimum color saturation must be met
to be called a ruby, otherwise the stone will be called a
pink sapphire.
[3]
This distinction between rubies and pink sapphires is relatively new,
having arisen sometime in the 20th century. If a distinction is made,
the line separating a ruby from a pink sapphire is not clear and highly
debated.
[5] As a result of the difficulty and subjectiveness of such distinctions, trade organizations such as the
International Colored Gemstone Association (ICGA) have adopted the broader definition for ruby which encompasses its lighter shades, including pink.
[6][7]
Natural occurrence
The
Mogok Valley in Upper
Myanmar
(Burma) was for centuries the world's main source for rubies. That
region has produced some of the finest rubies ever mined, but in recent
years very few good rubies have been found there. The very best color in
Myanmar rubies is sometimes described as "pigeon's blood." In central
Myanmar, the area of Mong Hsu began producing rubies during the 1990s
and rapidly became the world's main ruby mining area. The most recently
found ruby deposit in Myanmar is in Namya (Namyazeik) located in the
northern state of
Kachin.
Rubies have historically been mined in
Thailand, the
Pailin and
Samlout District of
Cambodia,
Burma,
India,
Afghanistan,
Australia,
Namibia,
Colombia,
Japan,
Scotland,
Brazil and in
Pakistan. In
Sri Lanka, lighter shades of rubies (often "pink sapphires") are more commonly found. After the
Second World War ruby deposits were found in
Tanzania,
Madagascar,
Vietnam,
Nepal,
Tajikistan, and
Pakistan.
[8]
A few rubies have been found in the
U.S. states of
Montana,
North Carolina,
South Carolina and
Wyoming. While searching for aluminous schists in Wyoming, geologist
Dan Hausel noted an association of
vermiculite with ruby and sapphire and located six previously undocumented deposits.
[9]
More recently,
large ruby deposits have been found under the receding ice shelf of
Greenland.
[citation needed]
Republic of Macedonia is the only country in mainland Europe to have naturally occurring rubies. They can mainly be found around the city of
Prilep. Macedonian ruby has a unique raspberry color. The ruby is also included on the Macedonian Coat of Arms.
In 2002 rubies were found in the
Waseges River area of
Kenya. There are reports of a large deposit of rubies found in 2009 in
Mozambique, in
Nanhumbir in the
Cabo Delgado district of
Montepuez.
[10]
Spinel,
another red gemstone, is sometimes found along with rubies in the same
gem gravel or marble. Red spinel may be mistaken for ruby by those
lacking experience with gems. However, the finest red spinels can have a
value approaching that of the average ruby.
[11]
Factors affecting value
Diamonds are graded using criteria that have become known as the four
Cs, namely color, cut, clarity and carat weight. Similarly natural
rubies can be evaluated using the four Cs together with their size and
geographic origin.
Color: In the evaluation of colored gemstones, color is the most important factor. Color divides into three components:
hue,
saturation and
tone.
Hue refers to "color" as we normally use the term. Transparent
gemstones occur in the following primary hues: red, orange, yellow,
green, blue, violet. These are known as
pure spectral hues.
[12]
In nature, there are rarely pure hues, so when speaking of the hue of a
gemstone, we speak of primary and secondary and sometimes tertiary
hues. In ruby, the primary hue must be red. All other hues of the gem
species corundum are called sapphire. Ruby may exhibit a range of
secondary hues. Orange, purple, violet and pink are possible.
-
A naturally occurring ruby crystal
-
Natural ruby with inclusions
-
-
The finest ruby is best described as being a vivid medium-dark toned
red. Secondary hues add an additional complication. Pink, orange, and
purple are the normal secondary hues in ruby. Of the three, purple is
preferred because, firstly, the purple reinforces the red, making it
appear richer.
[12]
Secondly, purple occupies a position on the color wheel halfway between
red and blue. In Burma where the term 'pigeon blood' originated, rubies
are set in pure gold. Pure gold is itself a highly saturated yellow.
When a purplish-red ruby is set in yellow, the yellow neutralizes its
complement blue, leaving the stone appearing to be pure red in the
setting.
[citation needed]
Treatments and enhancements
Improving the quality of gemstones by treating them is common
practice. Some treatments are used in almost all cases and are therefore
considered acceptable. During the late 1990s, a large supply of
low-cost materials caused a sudden surge in supply of heat-treated
rubies, leading to a downward pressure on ruby prices.
Improvements used include color alteration, improving transparency by
dissolving rutile inclusions, healing of fractures (cracks) or even
completely filling them.
The most common treatment is the application of heat. Most, if not
all, rubies at the lower end of the market are heat treated on the rough
stones to improve color, remove
purple tinge, blue patches and silk. These heat treatments typically occur around temperatures of 1800 °C (3300 °F).
[13]
Some rubies undergo a process of low tube heat, when the stone is
heated over charcoal of a temperature of about 1300 °C (2400 °F) for 20
to 30 minutes. The silk is only partially broken as the color is
improved.
Another treatment, which has become more frequent in recent years, is
lead glass filling. Filling the fractures inside the ruby with
lead glass
(or a similar material) dramatically improves the transparency of the
stone, making previously unsuitable rubies fit for applications in
jewelry.
[14] The process is done in four steps:
- The rough stones are pre-polished to eradicate all surface impurities that may affect the process
- The rough is cleaned with hydrogen fluoride
- The first heating process during which no fillers are added. The
heating process eradicates impurities inside the fractures. Although
this can be done at temperatures up to 1400 °C (2500 °F) it most likely
occurs at a temperature of around 900 °C (1600 °F) since the rutile silk
is still intact.
- The second heating process in an electrical oven with different
chemical additives. Different solutions and mixes have shown to be
successful, however mostly lead-containing glass-powder is used at
present. The ruby is dipped into oils, then covered with powder,
embedded on a tile and placed in the oven where it is heated at around
900 °C (1600 °F) for one hour in an oxidizing atmosphere. The orange
colored powder transforms upon heating into a transparent to
yellow-colored paste, which fills all fractures. After cooling the color
of the paste is fully transparent and dramatically improves the overall
transparency of the ruby.[15]
If a color needs to be added, the glass powder can be "enhanced" with
copper or other metal oxides as well as elements such as sodium,
calcium, potassium etc.
The second heating process can be repeated three to four times, even applying different mixtures.
[16]
When jewelry containing rubies is heated (for repairs) it should not be
coated with boracic acid or any other substance, as this can etch the
surface; it does not have to be "protected" like a diamond.
The treatment can easily be determined using a 10x loupe and
determination focuses on finding bubbles either in the cavities or in
the fractures that were filled with glass.
[17]
Synthetic and imitation rubies
Artificial ruby under a normal light (top) and under a green laser light (bottom). Red light is emitted
In 1837 Gaudin made the first synthetic rubies by fusing potash
alum at a high temperature with a little chromium as a pigment. In 1847
Ebelmen made white sapphire by fusing
alumina in boric acid. In 1877 Frenic and Freil made crystal
corundum from which small stones could be cut. Frimy and
Auguste Verneuil manufactured artificial ruby by fusing BaF
2 and Al
2O
3 with a little chromium at
red heat. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this
flame fusion process.
[18]
By 1910, Verneuil's laboratory had expanded into a 30 furnace
production facility, with annual gemstone production having reached
1,000 kilograms (2,000 lb) in 1907.
Other processes in which synthetic rubies can be produced are through Czochralski's
pulling process, flux process, and the
hydrothermal process.
Most synthetic rubies originate from flame fusion, due to the low costs
involved. Synthetic rubies may have no imperfections visible to the
naked eye but magnification may reveal curves,
striae
and gas bubbles. The fewer the number and the less obvious the
imperfections, the more valuable the ruby is; unless there are no
imperfections (i.e., a "perfect" ruby), in which case it will be
suspected of being artificial.
Dopants are added to some manufactured rubies so they can be identified as synthetic, but most need
gemological testing to determine their origin.
Synthetic rubies have technological uses as well as gemological ones. Rods of synthetic ruby are used to make
ruby lasers and
masers. The first working laser was made by
Theodore H. Maiman in 1960
[19] at
Hughes Research Laboratories in
Malibu, California, beating several research teams including those of
Charles H. Townes at
Columbia University,
Arthur Schawlow at
Bell Labs,
[20]
and Gould at a company called TRG (Technical Research Group). Maiman
used a solid-state light-pumped synthetic ruby to produce red laser
light at a wavelength of 694 nanometers (nm). Ruby lasers are still in
use. Rubies are also used in applications where high hardness is
required such as at wear exposed locations in modern mechanical
clockworks, or as scanning probe tips in a
coordinate measuring machine.
Imitation rubies are also marketed. Red
spinels, red
garnets,
and colored glass have been falsely claimed to be rubies. Imitations go
back to Roman times and already in the 17th century techniques were
developed to color foil red—by burning scarlet wool in the bottom part
of the furnace—which was then placed under the imitation stone.
[21] Trade terms such as
balas ruby for red spinel and
rubellite for red
tourmaline
can mislead unsuspecting buyers. Such terms are therefore discouraged
from use by many gemological associations such as the Laboratory Manual
Harmonisation Committee (LMHC).
Records and famous rubies
- The Smithsonian's National Museum of Natural History
in Washington, D.C. has received one of the world's largest and finest
ruby gemstones. The 23.1 carats (4.62 g) Burmese ruby, set in a platinum
ring with diamonds, was donated by businessman and philanthropist Peter Buck
in memory of his late wife Carmen Lúcia. This gemstone displays a
richly saturated red color combined with an exceptional transparency.
The finely proportioned cut provides vivid red reflections. The stone
was mined from the Mogok region of Burma (now Myanmar) in the 1930s.[22]
- In 2007 the London jeweler Garrard & Co featured on their website a heart-shaped 40.63-carat ruby.[23]
- On December 13/14, 2011 Elizabeth Taylor's complete jewellery collection was auctioned by Christie's.
Several ruby-set pieces were included in the sale, notably a ring set
with an 8.24 ct gem that broke the 'price-per-carat' record for rubies
($512,925 per carat, i.e. over $4.2 million in total),[24] and a necklace[25] that sold for over $3.7 million.
- The Liberty Bell Ruby is the largest mined ruby in the world. It was stolen in a heist in 2011.[26]
Historical and cultural references
- In Job 28:18 and Proverbs 3:15, wisdom is more valuable than rubies. In Proverbs 31:10, a wife of noble character is worth more than rubies.
- An early recorded transport and trading of rubies arises in the literature on the North Silk Road of China, wherein about 200 BC rubies were carried along this ancient trackway moving westward from China.[27]
- Rubies have always been held in high esteem in Asian countries. They
were used to ornament armor, scabbards, and harnesses of noblemen in
India and China. Rubies were laid beneath the foundation of buildings to
secure good fortune to the structure.[28]
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