A ruby is a pink to blood-red colored gemstone,
a variety of the mineral corundum. 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.
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 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 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 and diamond in hardness. Ruby is α-alumina in which a small fraction of the
aluminium3+ ions are replaced by chromium3+ ions. Each Cr3+ is surrounded octahedrally
by six O2- ions. This crystallographic arrangement strongly affects each Cr3+, 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 Cr3+, it is re-emitted as red luminescence.
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” 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. 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. 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. As a result of the difficulty
and subjectiveness of such distinctions, trade organizations such as the International Colored
Gemstone Association have adopted the broader definition for ruby which encompasses its
lighter shades, including pink. Natural occurrence
The Mogok Valley in Upper Myanmar 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 located in the northern state of
Kachin. Rubies have historically been mined in Thailand,
the Pailin and Samlout District of Cambodia, Burma, India, Afghanistan and in Pakistan.
In Sri Lanka, lighter shades of rubies are more commonly found. After the Second World
War ruby deposits were found in Tanzania, Madagascar, Vietnam, Nepal, Tajikistan, and
Pakistan. 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.
More recently, large ruby deposits have been found under the receding ice shelf of Greenland.
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. 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.
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.
The color of rubies varies from vermilion to red. The most desired color is “pigeon’s
blood”, which is pure red with a hint of blue. If the color is too pink, the stone is a pink
sapphire. The same is true if it is too violet – it is a violet sapphire. The best rubies
and star rubies are bright red. Most rubies come from Burma and Thailand.
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. 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. 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. 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.
Set a purplish-red ruby in yellow and the yellow neutralizes its complement blue leaving
the stone appearing to be pure red in the setting.
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 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.
Some rubies undergo a process of low tube heat, when the stone is heated over charcoal
of a temperature of about 1300 °C 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
dramatically improves the transparency of the stone, making previously unsuitable rubies
fit for applications in jewelry. 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
it most likely occurs at a temperature of around 900 °C 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 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. 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. When jewelry containing rubies is
heated 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.
Synthetic and imitation rubies 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 BaF2 and Al2O3 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. By 1910, Verneuil’s laboratory had expanded into a 30 furnace production facility,
with annual gemstone production having reached 1,000 kilograms 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, 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 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, and Gould at a company called TRG. Maiman used a solid-state light-pumped synthetic
ruby to produce red laser light at a wavelength of 694 nanometers. 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. 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.
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 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 in the 1930s. In 2007 the London jeweler Garrard & Co featured
on their website a heart-shaped 40.63-carat ruby.
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, and a necklace 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.
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.
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.
See also List of minerals
Verneuil process References External links
International Colored Stone Association’s ruby overview page
Webmineral crystallographic and mineral info