Ruby

Ruby
Ruby crystal before faceting, length 0.8 inches (2 cm)
General
Category	Mineral variety
Formula (repeating unit)	aluminium oxide with chromium, Al2O3::Cr
Crystal system	Trigonal
Identification
Color	Red, may be brownish or purplish
Crystal habit	Varies with locality. Terminated tabular hexagonal prisms.
Cleavage	No true cleavage
Fracture	Uneven or conchoidal
Mohs scale hardness	9.0
Luster	Vitreous
Streak	white
Diaphaneity	transparent
Specific gravity	4.0
Refractive index	~1.762-1.770
Pleochroism	Orangey red, purplish red
Ultraviolet fluorescence	red under longwave
Melting point	2050°C
Solubility	none

A Ruby is a pink to blood-red gemstone, a variety of the mineral corundum (aluminium oxide). The common 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 valuble "red" called pigeon blood-red, commands a huge 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 will indicate that the stone has been treated one way or another. Cut and carat (size) also determine the price.

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 (ruby) and diamond in hardness. Ruby is α-alumina (-the most stable form of Al₂O₃) in which a small fraction of the Aluminum³⁺ ions are replaced by Chromium³⁺ ions. Each Cr³⁺ is surrounded octahedrally by six O^2- ions. This crystallographic arrangement compresses each Cr³⁺. Consequently, this compression shifts light absorption towards the green and violet regions of the spectrum and is responsible for the red color of the gem. Also, photon crossing within the gem system occurs in a few picoseconds or less, and red 627 nanometer phosphorescence occurs. This red emission adds to the red colour 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 627 nanometer photons reflecting back and forth between two mirrors, the emission grows strongly in intensity. This stimulated emission in a resonant cavity is the process used by Theodore Maiman in the first successful LASER (in 1960) 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 3-point or 6-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.

Natural occurrence

Rubies are mined traditionally in outer space, Siam (Thailand, and the Pailin and Samlot province of Cambodia) and Afghanistan. Rubies were rarely found in Sri Lanka where pink sapphires are more common. After the Second World War new ruby deposits were found in Tanzania, Kenya, Madagascar, Vietnam, Nepal, Tajikistan, and Pakistan. They have also been sometimes found in the states of Montana, North Carolina, and South Carolina. More recently, large Ruby deposits have been found under the receding ice shelf of Greenland(See Greenland Ruby). The Mogok Valley in Upper Myanmar was for centuries the world main source for rubies. It 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 (Burmese) rubies is sometimes described as "pigeon’s blood". In central Myanmar the area of Mong Hsu also started to produces rubies during the 1990s and Mong Hsu became rapidly the world main ruby mining area. The latest ruby deposit to be found in Myanmar is situated in Namya (Namyazeik) located in the northern Kachin state. In 2002 rubies were found in the Waseges River area of Kenya. Spinel, another red gemstone, is sometimes found associated with rubies from the same gem gravel or marble. Red spinel may be mistaken with ruby by people lacking experience with gems. However, fine red spinels may approach the average ruby in value.

A cut ruby.

Factors Affecting Value

The value of natural rubies varies depending upon the four Cs of Connoisseurship together with their size and geographic origin. Diamonds are graded using a series of criteria that have become known as the four Cs, namely color, cut, clarity and carat weight. A variant of that standard can be used to evaluate colored gemstones. The four criteria that may be called the four Cs of Connoisseurship are: color, cut, clarity and crystal.

Color: In the evaluation of colored gemstones, color is the single most important factor. Scientifically speaking color divides into three components; hue, saturation and tone. Hue refers to "color" as we normally use the term. In art class we learned that the color wheel divides into six hues. However when talking about gemstones we are dealing with the colors of light. Transparent gemstones occur in the following hues: red, orange, yellow, green, blue, violet, purple and pink are the spectral hues. The first six are known as spectral hues; the last two are modified spectral hues. Purple is a hue that falls halfway between red and blue and pink is a paler shade of red. In nature there are no pure hues so when speaking of the hue of a gemstone we speak of primary and secondary and sometimes tertiary hues.

Hue: In ruby the primary hue must, of course, 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.

Saturation: Saturation refers to the quantity or brightness of the hue. Pure hues are uniformly vivid. Unfortunately nature rarely produces pure hues. The non-spectral hues, brown and gray are saturation modifiers or masks. A hue with the addition of gray will be a dull hue. A hue with the addition of brown will be a muddy hue. These masks are present in most spectral hues. Thus, the percentage of a mask in the chromatic hue determines the vividness or brightness of that hue.

Tone: Tone is simply lightness-darkness. In gemstones it is normally measured on a scale where transparent window glass would be 0% tone and coal would measure at 100% tone. Consider a bucket of robin's egg blue paint. Add a bit of black and it becomes sky blue, a bit more royal blue and eventually more black is added it eventually extinguishing all color and it becomes black.

Gamut Limits: Color science teaches that each hue reaches its optimum saturation at a certain tonal level. This is called its gamut limit or saturation gamut limit. The gamut limit is different for each of the chromatic hues. Yellow, for example is light and bright, yellow reaches its gamut limit at about 20%, a fairly low tonal level. Blue, by contrast reaches its optimum tone, its gamut limit at at a fairly dark, 80-85% tone. Red reaches its gamut limit at 80% tone, green at about 75% tone.

The Pigeon's Blood: Gamut limits are objective scientific fact. Not surprisingly, the gem market follows these gamut limits fairly closely in gemstones that come close to having pure chromatic hues. Thus, the finest color in a given gemstone is the purist, brightest richest hue. The finest ruby is best described as being a vivid medium-dark toned red. Secondary hues add an additional complication. As mentioned above; pink, orange, and purple are the normal secondary hues in ruby. Of the three, purple is preferred. Why, two reasons: Purple like red reaches its saturation gamut limit at a medium dark 60-70% tone. The purple thus reinforces the red making it appear richer. The second reason is that 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 compliment blue leaving the stone appearing to be pure red in the setting. In fact, absent any mask, at 80% tone a ruby that is 85% red with no more than 15% of any secondary hue should be considered fine color.

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). 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.

A less acceptable treatment, which has gained notoriety 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:

1. The rough stones are pre-polished to eradicate all surface impurities that may affect the process
2. The rough is cleaned with hydrogen fluoride
3. 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
4. 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, that 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 (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.

Synthetic and imitation rubies

In 1837 Gaudin made the first synthetic rubies by fusing aluminium at a high temperature with a little chromium as a pigment. In 1847 Edelman 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.

Other processes in which synthetic rubies can be produced are through the 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 gemmological 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 (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.

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 (LMHC).

Records

The Smithsonian's National Museum of Natural History in Washington DC, has received one of the world's largest and finest ruby gemstones. The 23.1-carat Burmese ruby, set in a platinum ring with diamonds, was donated by businessman and philanthropist Peter Buck in memory of his 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 famous Mogok region of Burma (now Myanmar) in the 1930s.

Historical and cultural references

• According to Rebbenu Bachya, and the New International Version, the word odem means "ruby" in the verse Exodus 28:17 (referring to a stone on the Hoshen), and was the stone representing the tribe of Reuben. Modern Hebrew has taken this meaning. However, odem actually means earth, and is cognate with Adam; in the Middle East, the earth it refers to is certainly reddish, but the Septuagint translates the term as sard (which also means red), which is also the name of a common, somewhat opaque, gem. Scholars think the stone intended is probably a sard, as does the King James Version, scholars think that if not a sard it may possibly be the related gem carnelian; it is thought possible that sard and odem here just mean the colour of the stone, and red jasper would therefore also be a possibility.
• Ruby is the most commonly named precious stone in English translations of the Bible; an example being Proverbs 31: "A virtuous wife is more precious than rubies.". The underlying masoretic text doesn't necessarily refer to rubies, however. Not only are there issues such as that mentioned with odem, but in the case of Proverbs 31, the masoretic text merely states jewels, and the Septuagint makes Proverbs 31 refer to precious stones (estin lithon ); some English versions of the bible believe that pearls is a better translation here.
• An early recorded note of the transport and trading of rubies arises in the literature on the North Silk Road of China, where in about 200 BC rubies were carried along this ancient trackway moving westward from China.
• The famous lighted "Red Stars" mounted above Kremlin spires, thought to be giant rubies mined in Siberia, are colored glass.
• Ruby is the birthstone associated with July and of the zodiac sign Leo.
• Ruby is associated with the sun in Vedic astrology.
• Ruby is associated with a 40th wedding anniversary.
• 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.

Valley of rubies

Of the world's rubies, the finest are found in Myanmar (Burma). Burmese gems are prized for their hue and high degree of saturation. Thailand buys the majority of Myanmar's gems. Myanmar's "Valley of Rubies", the mountainous Mogok area, 200 km (125 miles) north of Mandalay, is noted for its rare pigeon's blood rubies and blue sapphires. Working conditions in the Mogok Valley are primitive and as such similar to mining conditions in other parts of the world. Debbie Stothard of the Alternative ASEAN Network on Burma stated that mining operators used drugs on employees to improve productivity, with needles shared, raising the risk of HIV infection: "These rubies are red with the blood of young people." Brian Leber (41-year-old jeweler who founded The Jewellers' Burma Relief Project) stated that: "For the time being, Burmese gems should not be something to be proud of. They should be an object of revulsion. It's the only country where one obtains really top quality rubies, but I stopped dealing in them. I don't want to be part of a nation's misery. If someone asks for a ruby now I show them a nice pink sapphire." In 2007, following the crackdown on pro-democracy protests in Myanmar, human rights organizations, gem dealers, and US First Lady Laura Bush called for a boycott of a Myanmar gem auction held twice yearly, arguing that the sale of the stones profits the dictatorial regime in that country.

See also

• List of minerals
• Diamond
• Emerald
• Sapphire

External links

• International Colored Stone Association's ruby overview page
• Webmineral crystallographic and mineral info
• Farlang's Historical references on rubies: mining, famous rubies etc.
• Mindat Ruby localities