Natural ruby crystals from Winza, Tanzania
|aluminium oxide with chromium, Al2O3:Cr|
|Crystal class||Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
|Color||Near colorless through pink through all shades of red to a deep crimson|
|Crystal habit||Terminated tabular hexagonal prisms.|
|Cleavage||No true cleavage|
|Mohs scale hardness||9.0|
|Luster||Subadamantine, vitreous, pearly (on partings)|
|Specific gravity||3.97 – 4.05|
|Optical properties||Uniaxial -|
|Pleochroism||Strong: purplish-red – orangy-red|
|Ultraviolet fluorescence||red under longwave|
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, together with amethyst, sapphire, emerald, and diamond. They word ruby comes from ruber, Latin for red. The color of a ruby is due to the element chromium.
The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value. The brightest and most valuable "red" called blood-red or "pigeon blood", 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. Ruby is the traditional birthstone for July and is usually more pink than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies. The world's most expensive ruby is the Sunrise 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 moissanite falling somewhere in between corundum (ruby) and diamond in hardness. Sapphire, ruby, and pure corundum are α-alumina, the most stable form of Al2O3, in which 3 electrons leave each aluminum ion to join the regular octahedral group of six nearby O ions; in pure corundum this leaves all of the aluminum ions with a very stable configuration of no unpaired electrons or unfilled energy levels, and the crystal is perfectly colorless.
When a chromium atom replaces an occasional aluminum atom, it too loses 3 electrons to become a chromium ion to maintain the charge balance of the Al2O3 crystal. However, the Cr ions are larger and have electron orbitals in different directions than aluminum. The octahedral arrangement of the O ions is distorted, and the energy levels of the different orbitals of those Cr ions are slightly altered because of the directions to the O ions. Those energy differences correspond to absorption in the ultraviolet, violet, and yellow-green regions of the spectrum.
If one percent of the aluminum ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem. Additionally, absorption at any of the above wavelengths stimulates fluorescent emission of 694-nanometer-wavelength red light, which adds to its red color and perceived luster.
After absorbing short-wavelength light, there is short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs. If 694-nanometer photons pass through the crystal during that time, they can stimulate more fluorescent photons to be emitted in-phase with them, thus strengthening the intensity of that red light. By arranging mirrors or other means to pass emitted light repeatedly through the crystal, a ruby laser in this way produces a very high intensity of coherent red light.
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. These days, almost all rubies are treated in some form, with heat treatment being the most common practice. Untreated rubies of high 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.
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. Drawing a distinction between rubies and pink sapphires is relatively new, having arisen sometime in the 20th century. Often, the distinction between ruby and pink sapphire is not clear and can be debated. 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.
The Mogok Valley in Upper Myanmar (Burma) was for centuries the world's main source for rubies. That region has produced some very fine rubies, but in recent years few good rubies have been found there. 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.
Historically, rubies have also been mined in Thailand, in the Pailin and Samlout District of Cambodia, as well as in Afghanistan, Australia, Brazil, Colombia, India, Namibia, Japan, and Scotland; after the Second World War ruby deposits were found in Madagascar, Nepal, Pakistan, Tajikistan, Tanzania, and Vietnam. In Sri Lanka, lighter shades of rubies (often "pink sapphires") are more commonly found. 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. A few rubies have been found in the U.S. states of Montana, North Carolina, South Carolina and Wyoming.
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.
Rubies, as with other gemstones, are graded using criteria known as the four Cs, namely color, cut, clarity and carat weight. Rubies are also evaluated on the basis of their 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 pure spectral hues of red, orange, yellow, green, blue, violet. 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. Ruby is defined to be red. All other hues of the gem species corundum are called sapphire. Ruby may exhibit a range of secondary hues, including orange, purple, violet, and pink.
A naturally occurring ruby crystal
Natural ruby with inclusions
Rubies set in jewelry
A cut pink ruby
The finest ruby is 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 it reinforces the red, making it appear richer. Purple also occupies a position on the color wheel halfway between red and blue. 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.
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 rubies at the lower end of the market are heat treated 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 partially broken, and 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. The process is done in four steps:
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.
The treatment can identified by noting bubbles in cavities and fractures using a 10x loupe.
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 (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. 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. 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).
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العربية ياقوت ▪ Aragonés Rubí ▪ Asturianu Rubín ▪ Azərbaycanca Yaqut ▪ Bahasa Banjar Mirah delima ▪ Bân-lâm-gú Âng-pó-chio̍h ▪ Беларуская Рубін ▪ Беларуская (тарашкевіца) Рубін ▪ Български Рубин ▪ Català Robí ▪ Čeština Rubín ▪ Cymraeg Rhuddem ▪ Dansk Rubin ▪ Deutsch Rubin ▪ Eesti Rubiin ▪ Ελληνικά Ρουμπίνι ▪ Español Rubí ▪ Esperanto Rubeno ▪ Euskara Errubi ▪ فارسی یاقوت سرخ ▪ Français Rubis ▪ Gaeilge Rúibín ▪ Galego Rubí ▪ गोंयची कोंकणी / Gõychi Konknni माणिक ▪ 客家語/Hak-kâ-ngî Fùng-pó-sa̍k ▪ 한국어 루비 ▪ Հայերեն Սուտակ ▪ हिन्दी कुरुविंद ▪ Hrvatski Rubin ▪ Ido Rubino ▪ Bahasa Indonesia Mirah delima ▪ Interlingua Rubino ▪ Italiano Rubino ▪ עברית אודם ▪ ಕನ್ನಡ ಮಾಣಿಕ್ಯ (ರೂಬಿ) ▪ ქართული ლალი ▪ Kaszëbsczi Rubin ▪ Қазақша Рубин ▪ Latina Carbunculus ▪ Latviešu Rubīns ▪ Lëtzebuergesch Rubin ▪ Lietuvių Rubinas ▪ മലയാളം മാണിക്യം (നവരത്നം) ▪ मराठी माणिक ▪ مصرى ياقوت ▪ Bahasa Melayu Delima (permata) ▪ မြန်မာဘာသာ ပတ္တမြား ▪ Nederlands Robijn (korund) ▪ नेपाल भाषा माणिक्कम् (सन् १९९६या संकिपा) ▪ 日本語 ルビー ▪ Norsk bokmål Rubin ▪ Norsk nynorsk Rubin ▪ Oʻzbekcha/ўзбекча Yoqut ▪ ਪੰਜਾਬੀ ਰੂਬੀ ▪ پنجابی یاقوت ▪ پښتو ياقوت ▪ Polski Rubin ▪ Português Rubi ▪ Română Rubin ▪ Runa Simi Puka umiña ▪ Русский Рубин ▪ संस्कृतम् माणिक्यम् ▪ Scots Ruby ▪ සිංහල රතුකැට ▪ Simple English Ruby ▪ Slovenčina Rubín ▪ Slovenščina Rubin ▪ کوردیی ناوەندی یاقووت ▪ Српски / srpski Рубин ▪ Srpskohrvatski / српскохрватски Rubin ▪ Suomi Rubiini ▪ Svenska Rubin ▪ தமிழ் மாணிக்கம் ▪ తెలుగు కెంపు ▪ ไทย ทับทิม (อัญมณี) ▪ Тоҷикӣ Ёқут ▪ Türkçe Yakut ▪ Українська Рубін ▪ اردو یاقوت ▪ ئۇيغۇرچە / Uyghurche قىزىل ياقۇت ▪ Tiếng Việt Hồng ngọc ▪ ייִדיש רובין ▪ 粵語 紅寶石 ▪ 中文 红宝石 ▪