Buyers, sellers, investors, syndicates. Gems and Gem mines for sale Nordic Partnership + 44 0207 193 3604
Sunday, September 21, 2014
This article has been reprinted with permission from the American Gemological Laboratories (AGL).
Originally Published on the Rapaport Diamond Report, July, 2010 / Vol. 33, No. 07
By Christopher P. Smith, American Gemological Laboratories AGL
Through the ages, blue sapphires have been one of the most highly revered, sought-after and valued gems. This tradition continues today, as evidenced by the recent, June 1, 2010, Christie’s Hong Kong auction of a remarkable bracelet containing seven Kashmir sapphires that sold for a record-breaking $6,915,624. Historically, gems, including sapphires, were a primary expression of the wealth of the ruling class, demonstrating the power of rulers, the opulence of their courts and the richness of their nations’ treasure troves to fund exploration and colonization, as well as wars. Gems also have been used throughout time as an inheritance, in order to transfer wealth from one generation to another. In modern society, sapphires adorn celebrities, inspire jewelry designers and continue to be an expression of love and devotion.
Cartier sapphire bracelet circa 1960
The blue color of sapphire may be found in many evocative shades, ranging from the soft pastel of cerulean to richly saturated indigos. Ancient lore endowed sapphire with many admirable traits, such as friendship and loyalty, as well as the ability to protect the wearer of the gem. Today, in contrast to the fiery, passionate emotions associated with the ruby’s red color, the blue of sapphire is linked to truth, compatibility, commitment and mutual understanding. Sapphire is the birthstone for the month of September and, more recently, it has become the stone of choice for many engagement rings.4
Blue sapphire deposits is widely distributed in a variety of geologic conditions, allowing gem-quality stones to be unearthed in greater amounts and in significantly greater sizes than the other members of the corundum family, including the ruby and other color varieties of sapphires, with the occasional exception of large yellow sapphires. Exceedingly rare, fine-quality blue sapphires of several hundred carats have been found and can be seen in a variety of museums, crown jewels and private collections. Some of the most famous examples are the 563.35-carat Star of India, which resides in the American Museum of Natural History, the 423-carat Logan sapphire, housed at the Smithsonian, and the 100-plus-carat Stuart sapphire, incorporated in the British Crown jewels on display in the Tower of London.5’6
THE IMPORTANCE OF ORIGIN
The origin of a gemstone such as Burma for rubies, Colombia for emeralds and Brazil for copper-bearing tourmalines can have a profound impact on its market value. In the case of sapphires, the most highly reputed and valued stones bear the pedigree of Kashmir and Burma. Top-quality sapphires from these sources typically achieve the highest per-carat price at auction.7,8
Very fine sapphires also come from the islands of Sri Lanka (Ceylon) and Madagascar. Sapphires from other sources, such as Pailin, Cambodia, and Yogo Gulch, Montana, have experienced periods of heightened popularity and remain sought after by knowledgeable connoisseurs in various markets around the world.3,9
Although a number of other countries including Australia, China and Thailand are large producers of sapphire in terms of the total carats, mined, the overall quality and provenance of these stones does not command the same elevated status among connoisseurs.
Top-quality sapphires from Kashmir and Burma display the two types of colors that are most highly valued today.
PRODUCTION AND AVAILABILITY
Blue sapphires may form under either metamorphic or igneous conditions and are distributed across the globe (see “Key to Sapphire Chart”) .10,11
Ceylon, now known as Sri Lanka, an island off the Southern tip of India, is generally considered to be the world’s first source of sapphires and many other gems with a history that dates back more than 2,000 years.3,5,12 After Ceylon, the Kingdom of Pegu, also known Burma or Myanmar, has been a source of these azure-colored for more than 1,000 years.5,13 The original discoveries of gems in both locations are shrouded in lore and mystery; however, many of the most important sapphires in history maybe traced back to these two very prolific sources. A small, yet continuing, production of sapphires from these deposits is a tribute to the bounty of reserves in these locations, as well as to the benefits, to small-scale, artisanal mining that continues to be employed to unearth these gems.
By the mid-1600s, it appears that southern Thailand, and later the area now known as Cambodia, began to produce both sapphires and rubies. However, due to their use of mechanized mining, within the past few years, Thai deposits have become largely depleted, with little new production. 5,14
A significant discovery of sapphire in 1854 along the Eastern region of Australia has yielded, to date, a number of deposits in both Queensland and New South Wales.5 These deposits, which are hosted in alkali-basalt magmatic beds, still produce significant quantities of sapphire. They are so rich in sapphires that it has been suggested Australia has produced more of that gemstone by carat weight than any other source throughout history.5
Another important sapphire find in the state of Montana in 1865 brought an intriguing but limited source of sapphire to the American market.9 But it was an unexpected find of sapphire in the 1880s in a remote area of the Kashmir region, near the village of Sumjam in Northern India, which has had the most significant and lasting impact on the gemstone trade worldwide. Although the primary period of production for this source was a mere seven years, the yield was extraordinary.5,16 The majority of Kashmir sapphires in the market today were produced during that time. Since the initial discovery, sapphire mining has continued sporadically in the area on a very small scale, unearthing only a few good stones per year. However, to this day, there is the rare occurrence of a find that yields an important Kashmir stone.
With the exception of Kashmir, collectively Australia, Burma, Ceylon and Thailand/Cambodia were the world’s only significant suppliers of sapphire until well into the twentieth century. Beginning in the 1960s, and continuing to the present, a number of sapphire deposits worldwide are contributing small, yet steady, amounts to the global supply. In East Africa, several sapphire deposits have been uncovered in Tanzania, as well as Kenya and, most recently, Mozambique. China also has emerged as a major supplier of dark, commercial-quality sapphires.
For more than a century, the presence of sapphires was known in Madagascar. But it was only as a result of yet another fortuitous find of sapphires during 1996, near the town of Andranondambo, that the island nation became known as a major global source.17Madagascar’s fine sapphires occur in sizes and qualities to rival those of the most prestigious sources, including Sri Lanka, Burma and even Kashmir. However, as a newer source, Madagascar has not been able to command the same cachet for its sapphires in the marketplace as older, better-established sources.
In addition, periods of controversy and politics often have interrupted the production of sapphire from Madagascar and inhibited the country’s ability to provide a steady supply. Nonetheless, today Madagascar remains one of the most important suppliers of fine-quality blue sapphires and other gems for the gemstone and jewelry trade.18
Several other countries also possess reserves of sapphire that show promise. These include Brazil, Colombia, Laos, Nigeria, Rwanda, Vietnam, Nepal and Pakistan.5,19,20,21
TURNING UP THE HEAT
Much of the gem-quality sapphire available in the market today is heated. Prior to the early 1970s, sapphire heating was restricted to the age-old blow-pipe method and an open flame. However, a chance discovery demonstrated how pale-blue-to-white sapphire could be heated at higher temperatures, under certain conditions, to dramatically improve the color and clarity of a piece of rough. The story of this discovery has become near-legend and varies in its telling, depending on the source. Some say it was a Swiss company that discovered the secret.5 However, most tales generally revolve around an accidental fire that took place in Chantaburi, Thailand, at the house of an important gem merchant. In the aftermath of the fire, sifting through the ashes of what had been his home and place of business, the Thai gem dealer found hordes of previously discarded low-quality sapphire that had miraculously become crystal clear, with a vibrant blue color, in the heat of the fire. This tale maybe fact or fiction, but it is fact that during the 1970s Thailand became known for low-grade sapphire, locally known as geuda, to produce sapphires of a strong blue color. Today there are many sub-classifications of geuda such as, milky, silky, diesel and others – which the treaters distinguish and heat under specific conditions.5,22
This heating can improve the clarity of a stone by dissolving fine inclusions that reduce transparency and greatly enhance the color by enabling the pairing of iron and titanium atoms to occur, which is the mechanism responsible for the blue color in sapphire
(see “Key to Sapphire Chart”).
Blue sapphire, regardless of its origin, whether natural-color or heated, continues to be one of the most popular colored stones sold at retail. Although top, gem-quality sapphires from the world’s premier sources are becoming increasingly rare, newer deposits and improved heating procedures help to meet demand and promise additional, sustainable supplies for many years to come.
Deposits of gem-quality sapphire can be found in approximately 20 countries around the world. These deposits are not randomly distributed, but their locations are closely linked to major geologic events and plate tectonics. The preponderance of sapphire deposits occur in three geologic settings.
The first involves the orogeny or mountain-building event that is responsible for the creation of the Himalayan mountain chain— and several other associated mountain ranges— which occurred when the Eurasian and Indian plates collided approximately 55 million years ago. These sapphire deposits extend from the remote areas of Afghanistan, Pakistan and Kashmir in Northern India, through Nepal into Burma and across to Vietnam. Many of these areas are producers of gem-quality ruby as well. (See “Inside Rubies,” published in Rapaport Diamond Report, December 5, 2008,Vol. 31, No. 47.)
The second major geologic event that is responsible for a number of sapphire deposits, as well as other gems, includes the Pan-African orogeny, estimated to have taken place approximately 800 million to 450 million years ago. This mountain-building process occurred prior to the separation of a large landmass known as Gondwana, when what are presently Eastern Africa, Sri Lanka, Madagascar and India were all united. This event is responsible for the sapphire deposits of Sri Lanka, Madagascar, Kenya, Tanzania, Mozambique and others.
A number of sapphire sources are also associated with eruptive events at the continental margin of the Pacific plate’s western edge. Along this subduction zone, erupting alkali-basalt magmas have intersected sapphire occurrences and transported them to the surface. These sapphire sources include Thailand, Cambodia, Laos, Southern Vietnam and Australia. A number of other countries also have alkali basalt-hosted deposits— commonly referred to as basaltic deposits— that produce sapphires, including China, Colombia, Madagascar, Nigeria and Rwanda.
A seemingly random occurrence of sapphire from a different type of magmatic-related environment hosted by a lamprophyre rock has been found in the state of Montana, in the U.S.
THE GROWTH OF SAPPHIRE
Blue sapphire deposits found around the world have formed under either metamorphic or igneous/magmatic growth conditions. Metamorphism is when preexisting rocks are recrystallized without melting due to changes in temperature, pressure and/or chemical composition on a regional or local scale. A number of specific metamorphic environments are responsible for the growth of sapphires within individual countries or areas. Currently, it is generally accepted that the classical blue, green and yellow sapphires— sometimes referred to as BGY-series— recovered from magmatic or basaltic deposits, have not formed in the alkali-rich basaltic magma where they are found, but they do have an igneous origin. Researchers have proposed that corundum could crystallize directly from certain volatile-rich magmas that are silica-poor and alumina-rich rocks under restricted conditions present in the upper mantle.
Gem-quality sapphire deposits are typically categorized into two main groups: metamorphic and magmatic-related.
Metamorphic deposits: These sapphire deposits are primary sources, meaning these sapphires actually formed in the host rock in which they are found. The host rock is commonly a marble, schist or gneiss and can be associated with shear zones.
Magmatic-related deposits: These sapphires are found in magmas. Magmatic-eruptive events are technically only a secondary source because the erupting magma is merely the transport mechanism that brings the sapphires to the earth’s surface and not the host rock in which the sapphire formed. Since the sapphires are recovered at the surface in the magma flow, many people have incorrectly considered this type of deposit to be a primary source. However, it is more appropriately considered another kind of secondary deposit.
SOURCE TYPE TYPE CLASSIFICATON
A system of classification was developed by C. P. Smith, author of this article— while director of the Gubelin Gem Lab in Switzerland— that combines geologic and gemological considerations in order to compare sapphires with various properties and internal characteristics forming in similar growth environments. This system allows for sapphires of similar “types” but from different countries to be compared, as well as sapphires of different “types” to be distinguished from each other. This classification can also be applied to rubies. (See “Inside Rubies,” published in Rapaport Diamond Report, December 5, 2008,Vol. 31, No. 47.)
The classification system has two tiers. The first tier separates sapphires into three groups based on broad geologic formation scenarios. Two of the three groups contain stones that possess what are considered by experienced gemologists to be “classical” combinations of specific gemological features for metamorphic and magmatic-related sources. For the sapphires from metamorphic environments, referred to as Met, such features include a blue color that is primarily due to an intervalence charge transfer between iron (Fe) and titanium (Ti) —Fe2+↔ Ti4+ —with a lesser influence due to iron present as Fe and mineral inclusions such as calcite, apatite, mica and others. The blue of sapphires in this category, although it may be very saturated, is typically lighter or brighter than that of the magmatic- related group. Typical sources for the Met group include Burma (Mogok area), Ceylon (Sri Lanka), Madagascar (both Andranondambo and Ilakaka) and Kashmir in Northern India.
Sapphires from magmatic-related environments, referred to as Mag, have classical features that include a darker blue, often greenish, color and a relatively higher iron content. Common mineral inclusions include uranpyrochlore, ilmenite and niobite-columbite, as well as others. A telltale spectroscopic feature of sapphires in the Mag group includes an absorption attributed to the intervalence charge transfer between Fe2+ and Fe3+ Typical sources of sapphire in the Mag group include Australia, China, Thailand and Cambodia.
A third group designated as Met-Mag indicates those sources that have properties and characteristics outside of the classical ones for either Met or Mag. Such sapphires may be recovered from either a metamorphic or magmatic- related deposit. However, they are characterized by higher iron contents than are typical of Met sapphires and an absence of the Fe2+ ↔Fe3+ intervalence charge transfer in the absorption spectrum. Sapphire deposits that fall into the Met-Mag group include the state of Montana in the U.S., Tanzania— both Songea and the Umba Valley and Colombia.
A subset of sapphires from the classical magmatic-related— alkali basalt-hosted —deposits are actually formed under deep metamorphic conditions and were subsequently caught up in the same eruptive event that brought the igneous-formed sapphires to the surface. Such sapphires are sometimes referred to as “metamorphic-suite sapphires from magmatic sources,” and are also classified as Met-Mag due to their properties and characteristics.
The second tier of the classification system subdivides each of these three groups into four categories or “types” —Type I to Type IV— based on their dominant inclusion features and supported by various other features and data from advanced analytical techniques. Type I stones are typified by “silk” inclusions, generally needles of rutile. Type II are characterized by very fine-grained, zonal clouds of “dust” particles. Type III may be distinguished by various cross-hatch and flake-like inclusion patterns. Type IV are identified by zircon inclusions and/or crystals associated with thin films. Combinations of these types may occur when multiple features are encountered in a stone.
METHODS OF TREATMENT
As with most gem materials, a variety of treatments can be applied to improve the apparent quality of a sapphire. These range from relatively simple tech- niques to much more advanced methods.
Oiling: Filling of fissures is an enhancement method that dates back to antiquity. In non-heated sapphires, fissures may be filled with an oil or similar substance to reduce their visibility and improve the apparent clarity of the gem.
Heat only: Relatively low-tem-perature heating, such as the blow-pipe method, has been used for centuries. However, modern heating at relatively higher temperatures, using an oven and controlled environments, has been commercially used since about the mid-1970s. Heat alone can be used to modify the color of a sapphire; most commonly, sapphires are heated under reducing conditions to induce or improve the blue color. Additionally, the apparent clarity may be improved, as heating can also dissolve dense concentrations of rutile needles.
Heat +fissure healing: Although not as common a practice as with rubies, sapphires treated by heating combined with the use of fluxing agents are encountered in the market. In addition to modifying the color and dissolving rutile needles, fluxing agents can be used during the heating process to heal fissures. Remnants of flux become trapped inside the stone along the preexisting fissures. Another by- product of this process is that the flux may also fill surface-reaching cavities.
Heat + Ti- diffusion: The tita-nium diffusion of sapphire has been known and available in the industry since approximately the mid- to late-1980s. Under more extreme heating conditions higher temperatures and/or longer duration it is possible to diffuse elements, such as titanium, into the lattice of a sapphire. This will induce a shallow blue color at the surface of a stone.
Heat + Be-diffusion: The beryl- lium diffusion of sapphire took the industry by surprise in 2001. Under these more extreme heating conditions higher temperatures and/or longer duration it is also possible to diffuse beryllium into the lattice of a sapphire. This will induce, modify and/or remove color, depending on the heating conditions, inherent chemical composition of the stone and the amount of beryllium introduced.
Cobalt-glass fissure in-filling: In recent years, a method was developed to dramatically improve the apparent clarity of low-quality ruby by infusing it with a high refractive index lead glass. Consequently, a cobalt-blue colored glass has been used to induce a blue color in low-grade corundum and improve the apparent clarity.This treatment is primarily seen in low-quality beads.
Cobalt coating: Although not widely available, neon-colored blue corundum has been occasionally encountered in the gemstone trade that owes its color to an amorphous coating containing aluminum and cobalt. The color of these stones is unlike anything naturally encountered in sapphire.
(1) Christie’s Jewels Sale Fetches $60M in Hong Kong – http://www.rapnet.com/lang/zhtw/News/Newsltem.aspx?Articleid=31192
(2) Webster R. (1983) Gems: Their Sources, Descriptions and Identification. 4th Ed. Butterworth & Co. England 1006 pp.
(3) Gubelin E.J. (1975) The Color Treasury of Gemstones. English translation. SilvaVerlag, Zurich, pp.138.
(22) Themelis T. (2010) The Heat Treatment of Ruby and Sapphire. 2nd Ed. Ted Themelis publishing, Bangkok, Thailand, pp. 384.
Additional References for Key to Sapphire Chart:
Garnier V., Giuliani G., Ohnenstetter D., Schwarz D. (2004a) Les gisements de corindon: Classification et genese. Le Regne Mineral, No. 55, pp. 7-34 plus references.
Giuliani G., Ohnenstetter D., Gamier V., Fallick A.E., Rakotondrazafy M., Schwartz D. (2007) The Geology and Genesis of Gem Corundum Deposits, Chapter 2. Geology of Gem Deposits: Short Course Series, Vol. 37. Ed. Groat L.E., pp. 23-78.
Peucat ,t-J., Ruffault P. Fritsch E., Simonet C., Bouhnik-Le Coz M., Lasnier B. (2005) Un nouvel outil geochimique de renaissance des saphirs bleus basaltiques et metamorphiques: Le rapport Ga/Mg. Revue de Gemmologie a.f.g., No. 153, pp. 8-12.
Schwarz D. (1998) Aus Basalten, Marmoren and Pegmatiten: Spezielle Ursachen formten in der Erdkruste edle Rubine and Saphir. In C. Weise, ed., Rubin, Saphir & Korund; Schon, Hart, Selten, Kostbar, ExtraLapis, Vol. 15, pp. 5-9.
Smith C.P., McClure S.F., Shigley J.E., Notari F., Giuliani G. Van der Bogert C. (submitted) Corundum – Source Type Classification and Geographic Origin Declarations: Part 1. Gems & Gemology
Sutherland F.L., Schwarz D., Jobbins E.A., Coenraads R.R. Webb G. (1998) Distinctive gem corundum suites from discrete basalt fields: a comparative study of Barrington, Australia, and West Pailin, Cambodia, gemfields. Journal of Gemmology, Vol. 6, No. 2, pp. 65-85.