On the nature of rubies
Ruby is one variety of the mineral corundum. Another well-known variety is sapphire. Of the two, ruby is the most valuable.
With a rating of 9 on the Moh's scale of mineral hardness, it is the hardest stone next to diamond but still is only 1/140 as hard. The hardness of a ruby varies in different directions and this is a quality that lapidaries use to their advantage when cutting these red gemstones.
The amount of chrome affects the color of ruby, which can be found in various shades from pink to blood red. While it can be difficult to identify a ruby's origin by color, Burma is known for its exceptional rubies that can be the color of pigeon's blood. Rubies from Thailand often have a violet or brown hue while those from Sri Lanka are said to be tinted like a raspberry. Of all these delightful shades, pigeon's blood is most desirable to experts and connoisseurs.
Inclusions are common in rubies, but are not necessarily indicative of quality. They are helpful in identifying natural gemstones from synthetic. Rutile needles within a ruby can bring about a soft sheen called silk or, with the aid of skillful lapidary using the en cabochon cut, can create the cat's eye effect or a star with six rays (asterism).
On finding rubies and what to look for
Like many gemstones, rubies can be found all over the world but the most important producers are Burma, Thailand, Sri Lanka, and Tanzania. For hundreds of years, the most desirable stones have come from upper Burma near Mogok. Unimportant deposits are located in many areas, including North Carolina.
Rubies are evaluated according their color, clarity, cut, and carat weight.
Color. Color is the primary factor in determining a ruby's value. Fine specimens will be a rich, vibrant red to a somewhat purplish red. There are many poetic descriptive listings of the colors of ruby. More commonly used are (from highest value to lowest): pigeon's blood, ox blood, rabbit blood, and pink like a flower. The ancient Hindu Purana texts include: like the China rose, like blood, like saffron, like the resin of certain trees, like the seeds of the pomegranate, like red lead, like the red lotus, like the eyes of the Greek partridge or the Indian crane, like the interior of the half-blown red water lily.
Clarity. Inclusions can create desirable effects on a ruby (such silk or cat's eye). But if they affect the transparency or brilliance, the inclusions can also have a negative impact on the value. Some inclusions and surface-reaching fractures may pose a threat to the durability of a particular ruby.
Cut. Ruby's crystal structure determines its suitability for certain cuts. Common shapes include ovals and cushions with brilliant-cut crowns of kite-shaped and triangular facets and step-cut pavilions with concentric rows of rectangular or square facets.
Round, triangular, emerald-cut, pear, and marquise rubies are also available. These shapes are rare in larger sizes and higher qualities.
Pleochroism — the appearance of different colors in different crystal directions — is another factor that influences cut. In ruby it typically appears as red to purplish red in one crystal direction and orangey red in the other. Cutters can minimize the orangey red color by orienting the table facet perpendicular to the long crystal direction. Even so, it’s not always possible to orient a ruby for ideal color return because the potential loss of weight would be too great. (GIA)
Carat weight. Fine rubies over one carat are rare. In fact, large rubies are rarer than comparable diamonds and can be extremely expensive.
Filling. Stones with surface-reaching fractures or cavities can be filled with a variety of colorless materials including glass, resin, wax, and oil. Filling gemstones can enhance their appearance and stability. Rubies are most commonly filled with glass.
Heating. A ruby may be heated to achieve a variety of effects. It can remove a naturally occurring purplish hue leaving the gemstone a pure red. It can also help remove miniscule inclusions that interfere with a stone's transparency. On the other hand, heating may also cause inclusions to recrystallize and strengthen their effect. For example, a star ruby may be heated to enhance the asterism.
Rubies also can be dyed or subject to lattice infusion.
Historical notes and other interesting facts
400 carats. The largest cutable ruby, weighing 400 carats, was found in Burma. It was cut into three stones.
A ruby pedestal. In the Islamic story of the angel Atlas, not only did God appoint him to carry the world on his shoulders but he was also given a great ruby rock to stand upon.
Anthrax and carbuncles. Ancient Greeks called rubies anthrax, while Romans referred to them as carbuncules. During Shakespearean times, rubies were known as carbuncles.
By the light of a ruby. In Christian Rosenkreutz's Chymical Wedding, Venus's bed chamber is lit by rubies. It was a common belief that some rubies had the power to light a darkened room.
Blood ailments. For centuries, rubies were believe to cure diseases of the blood and to stop bleeding.
Warrior's stone. In many Asian and Malaysian countries, people believed that ruby protected a warrior from weapons of steel. To be effective, though, the ruby had to be inserted under the skin... the warrior's skin.
Not ruby but spinel. Before 1800, garnet and red spinel were also considered ruby. Many court jewels throughout history have featured red stones that were touted as rubies. In several cases the "rubies" turned out to be spinels, like the Black Prince's Ruby.
A ruby pig. Madame de Pompadour had a rather large ruby cut into the shape of pig, which she wore for good luck. It can be seen in the Louvre.
Rosser Reeves Star Ruby. Donated to the Smithsonian in 1965, the Rosser Reeves Star Ruby may be the largest and finest star ruby in the world. The Rosser Reeves Star Ruby is from Sri Lanka, but its early history is not known. When it was purchased by a gem dealer in London in the late 1950s, the ruby weighed 140 carats, but it was subsequently recut to center the star. Rosser Reeves, whose name it now bears, carried it around as a lucky stone, referring to it as his baby.
Ruby or glass? A quick way to test if a stone is really a ruby or a simply a bit of a glass is to put it in your mouth. A ruby will feel cold, rather like ice. This is due to its high level of thermal conductivity. Glass does not have this effect, it feels warmer since it is less thermally conductive.
Resources and further reading
The Gemological Institute of America's Gem Encyclopedia.
Gemstones of the World. By Walter Schuman. Our go to book at Grimballs.
Gemstones: Beauty, Lore, and Fascination. By Michael Babinski.
Jewels: A Secret History. By Victoria Finlay. An non-fiction adventure through time and place exploring the history of gems.