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Ruby and Sapphire


Red corundums are rubies. Blue corundums are sapphires. Trace elements produce their colors.


Article by: , Ph.D., GIA Graduate Gemologist


rubies

Rubies: The most desired variety of corundum is the ruby. The red color is produced by trace amounts of chromium in the mineral. These two beautiful rubies were mined in Madagascar. The one on the left is a 7 x 5 millimeter octagon that weighs about 1.32 carats. The one on the right is an 8 x 6 millimeter oval that weighs about 1.34 carats. Although Asia has been the traditional source of gem corundum for over one thousand years, Africa is poised to become a new primary source.

Ruby, Sapphire, and Fancy Sapphire

Most people don't realize that ruby and sapphire are both gems of the mineral corundum. Both of these gemstones have the same chemical composition and the same mineral structure. Trace amounts of impurities determine if a gem corundum will be a brilliant red ruby or a beautiful blue sapphire. It is surprising that "impurities" can produce such wonderful results!

Red and blue are just two of the many colors found in gem corundums. Trace amounts of other elements can produce brilliant yellow, orange, green, and purple gems. Red corundums are known as "rubies," blue corundums are known as "sapphires," and corundums of any other color are known as "fancy sapphires." Corundum is found in a spectrum of colors.




What Makes a Ruby?

Some gem-quality corundum contains trace amounts of chromium. A very small amount of chromium gives corundum a pink color. A little more chromium will increase the color saturation of the stone and produce a gem with a deeper red color. To be considered a "ruby," a corundum should have a color between orangey red and a slightly purplish red. The most desirable color is a pure vibrant red.

Ruby Treatments

Very few specimens of corundum have a natural color within the range required for a ruby. Very few also have the clarity required to produce a nice faceted stone. Long ago, people who prepared gem materials for cutting discovered that heating corundum crystals under controlled conditions can change or intensify their color. Heating can also cause inclusions to become less visible and improve the clarity of a gem.

Most rubies in the market today have been heated to improve their color and clarity. This heat treatment is normal and expected in the gem trade, but a seller should disclose the treatment to a buyer in advance of a sale.

Some rubies are heat treated with a fracture-filling material. These fillings might be flux, glass, polymer or another material that enters the fractures during the heating process. The fracture filling material remains in the gem, makes the fracture less visible, and makes the stone more durable - but it also adds some non-ruby material that will increase the weight of the gem. Small amounts of non-ruby material are often acceptable. However, if large amounts are injected or the ruby melts and mixes with non-ruby material, these gems should not be called "rubies." Why? Because anyone who buys them "by the carat" will be paying a significant portion of their cost for non-ruby material. These materials are really man-made composites, worth significantly less than a natural ruby or a heat-treated ruby.



Yogo Gulch Sapphire - Montana

Montana sapphire: The most widely known sapphire locality in North America is Yogo Gulch, Montana, famous for producing deep blue sapphires of excellent quality. Creative Commons photo of a gem from Barnes Jewelry, Helena, Montana, by Montanabw.

corundum as ruby, sapphire, and fancy sapphire

Corundum as ruby, sapphire, and fancy sapphire: Gem-quality corundum is a highly prized and valuable material. When it is bright red in color, it is called "ruby." When it is deep blue, it is called "sapphire." Gem-quality corundum of any other color is called "fancy sapphire." All of the stones in this photo were mined in Africa.

What Makes a Sapphire?

Trace amounts of iron and titanium can produce a blue color in corundum. Blue corundums are known as "sapphires." The name "sapphire" is used for corundums that range from light blue to dark blue in color. The blue can range from a violetish blue to a greenish blue. Stones in the middle of this range, with a rich blue color, are the most desirable.

Gem-quality corundum occurs in a wide range of other colors, including pink, purple, orange, yellow, and green. These stones are known as "fancy sapphires." It is surprising that a single mineral can produce gemstones of so many different colors.

When the color of a sapphire is any color other than blue, the color should be used as a preceding adjective to describe the stone. For example, pink sapphire, yellow sapphire, or green sapphire. Used alone, the word "sapphire" refers only to blue corundum.

Sapphire Treatments

The color and clarity of blue and fancy sapphires can be altered by heat, radiation, and other treatments. Sometimes pale, milky, cloudy, or translucent stones can be heated to yield stones that are transparent and bright blue in color. If such treatments have been used, sellers should disclose the types of treatments to potential buyers when the stone is presented for sale.

Some sapphires are treated by a process known as "lattice diffusion". In lattice diffusion, the sapphire is heated in the presence of a material that will release tiny metal ions. When the corundum is heated, its lattice expands enough that these tiny ions can diffuse into the gem and become trapped within the lattice when the corundum is allowed to cool. The trapped ion prevents the corundum from contracting to its normal dimensions, producing a defect that alters the color of light passing through the gem. Beryllium diffusion can produce orange, yellow and pink colors. Titanium diffusion can produce blue. Lattice diffusion treatment is considered to be a severe treatment that is sometimes hard to detect. Many people believe that corundum that has its color produced by lattice diffusion should not be called "sapphire".

ruby crystal

Ruby on marble: A ruby crystal on white marble from Jegdalek, Saroby, Afghanistan. This crystal is about 1.6 centimeters in length. Specimen and photo by Arkenstone / www.iRocks.com.

sapphire crystal

Sapphire crystal: A blue, doubly terminated, translucent sapphire crystal from Sri Lanka. Most crystal specimens like this have been transported by streams and show a greater amount of wear. This specimen has not been treated. Heat treatment would likely deepen the color, make it more uniform, and improve the clarity. This crystal is about five centimeters in length. Specimen and photo by Arkenstone / www.iRocks.com.

Popularity of Ruby and Sapphire

Ruby and sapphire are extremely popular gemstones. Virtually every jewelry store that features colored gemstones in jewelry will have a generous portion of their display dedicated to ruby and sapphire items. Ruby is the most popular red gemstone, and sapphire is the most popular blue gemstone.

The pie chart below shows the share of colored stone imports on a dollar value basis that went to the categories of sapphire, ruby, emerald, and all other gemstone varieties during the 2011 calendar year. It shows that sapphire and ruby were the second and third most imported colored stones during that year. A total of $282 million worth of sapphire was imported, and a total of $45 million worth of ruby was imported.

Imports of colored stones into the United States

Colored stone imports: This chart illustrates the popularity of sapphire and ruby in the United States. The pie represents all colored stones imported into the United States during 2011 on the basis of dollar value. As single gem varieties, sapphire and ruby hold major positions in the import market, accounting for over 35% of all colored stones imported and a total value of approximately $400 million. Data is from the United States Geological Survey Minerals Yearbook, April 2013. [5]

Although the pie chart does not include domestic colored stone production, it can be considered as nearly complete. The United States Geological Survey estimates that the value of the total domestic production of colored stones of all kinds in the United States during calendar year 2011 was only $11 million.

Related:   Emerald: The Most Popular Green Gem

synthetic star ruby

Synthetic star ruby: Laboratories have been able to mass-produce synthetic star corundum since the Lindy division of Union Carbide flooded the gem market with them in the 1950s and 1960s. This synthetic red corundum has a visible six-ray star and a faceted back to enhance the brightness of the stone.

star sapphire

Star sapphire: Some specimens of sapphire and ruby contain a very fine "silk" of fibrous inclusions that parallel the crystallographic axes of the mineral. When these stones are cut into cabochons with their c-axis penetrating their base at right angles, a six-rayed star can be seen floating on the surface of the cabochon. These are known as "star sapphires" or "star rubies," according to their color. Public domain photography by Mitchell Gore.

Mining Rubies and Sapphires

Most gem-grade corundum forms in metamorphic rocks, such as schist or gneiss; or in igneous rocks such as basalt or syenite. However, gem corundums are rarely mined from the rocks in which they form. Mining small gems from hard rock is possible, but it is very expensive, and many of the gems are broken during the mining process. Fortunately, corundum is very hard and resistant to weathering. In many areas, natural weathering and erosion have liberated the stones from their host rock, and carried them into streams over long periods of geologic time.

Today, the gems are mined from these stream sediments. Their high specific gravity relative to other sediment particles often causes currents to concentrate them in small placer deposits. Most rubies and sapphires are produced by washing the gravels of these stream deposits. This work is often done by hand because the deposits are small and irregular in shape and character. These deposits are often located in countries when wages are very low and artisanal mining is prevalent.

Noteworthy locations where gem-quality corundums have been produced include Myanmar, Thailand, Cambodia, Vietnam, India, Pakistan, Afghanistan, Sri Lanka, China, Australia, Madagascar, Kenya, Tanzania, Nigeria, and Malawi.

blue star sapphire

Black star sapphire: A black star sapphire 8 mm x 6 mm cabochon from Thailand. Inclusions within the stone align with the crystallographic axis to produce a six-ray silvery star. When the star is clearly visible and centered, as in this example, the base of the stone intersects the c-axis of the corundum crystal at 90 degrees. This stone has been heat treated to darken the stone and enhance the visibility of the star.

Ruby and Sapphire Information
[1] Gems: Their Sources, Descriptions and Identification: Michael O’Donoghue; Elsevier; sixth edition; 873 pages; 2006.

[2] Gemstones of the World: Walter Schumann; Sterling Publishing; fifth edition; 320 pages; 2013.

[3] Rubies and Sapphires: Fred Ward; Fred Ward Gem Books; 65 pages; 2010.

[4] Rock and Gem: Ronald L. Bonewitz; The Smithsonian Institution, Dorling Kindersley Publishing; 360 pages; 2008.

[5] 2011 Minerals Yearbook: Gemstones: Donald W. Olson; The United States Geological Survey; 22 pages; 2013.


Synthetic Corundum

Rubies and sapphires have been highly sought after in many parts of the world for over one thousand years. Deposits that produce high-quality stones of good color have attracted enormous amounts of attention and have been heavily exploited. As a result, buyers who need large quantities of quality stones are having a harder time finding them in the volumes needed for today's jewelry marketplace.

Let's imagine a jewelry manufacturer who wants to create enough matching ruby pendant, ring, and earring sets to supply a large jewelry chain with over 1000 stores and a busy internet site. This manufacturer will need at least four nice rubies for each matching set, multiplied by enough sets to supply over 1000 stores and a busy internet site.

This manufacturer will need millions of rubies, all color-matched into sets and all cut into calibrated shapes and sizes. On the production side, the labor needed to discover, mine, grade, cut, and polish these stones will be enormous. An enormous effort will also be needed on the manufacturing side just to find enough sellers to provide them, confirm their quality, negotiate prices, make large numbers of purchases, and deliver the stones to the manufacturing facility.

The problem of sourcing gemstones to meet the needs of a large manufacturing company is so much simpler if the stones themselves are manufactured. This is why laboratories capable of reliably producing synthetic rubies and sapphires of consistent size, color, grade, and appearance have found an important place in the gemstone market.

If you go shopping in the United States and look at the ruby and sapphire jewelry offered in many famous-name mall jewelry stores and department stores in the $50 to $500 price range, you will find that many of the jewelry items are made using "created" or "synthetic" rubies and sapphires.

The synthetic corundum in this jewelry is perfect in color, has wonderful clarity and is extremely attractive. Many shoppers see the lower price and better appearance of the synthetic materials when compared to natural stones of similar size and opt to purchase the synthetic. It is a logical choice based upon what appeals to the person and what they are willing to pay. They get great appearance at a lower price.

Natural gemstones are a finite resource that will become more difficult to obtain and more expensive over time. As a result, buyers will probably see more synthetic stones offered in most jewelry stores and should expect to see the price difference between synthetic stones and natural stones of similar size, color, and quality become greater in the future.

Many consumers prefer natural stones, and many also make the decision to purchase synthetic stones. That follows the often-given advice of "buy what you like and can afford."



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