Ruby Buying Guide title image
By Richard W. Hughes

The term ruby is reserved for corundums of a red color, with other colors called sapphire. In Asia, pink corundums are also considered rubies. Outside of Asia, such gems are generally termed pink sapphires.

For ruby, the intensity of the red color is the primary factor in determining value. The ideal stone displays an intense, rich crimson without being too light or too dark. Stones which are too dark and garnety in appearance, or too light in color, are less highly valued. The finest rubies display a color similar to that of a red traffic light.

Rough & Cut Rubies photo image
Fine ruby specimens. The crystals are from, from left to right, Afghanistan, Vietnam and Tanzania, while the two faceted gems are from Mogok, Burma.
(Photo: Harold & Erica Van Pelt; Gems: Pala International)

Rubies generally look best viewed with incandescent light or daylight (particularly around midday). Avoid fluorescent tubes, which have virtually no output in the red end of the spectrum, and so cause ruby to appear grayish.

In terms of clarity, ruby tends to be less clean than sapphire. Buyers should look for stones which are eye-clean, i.e., with no inclusions visible to the unaided eye. In the case of some rubies, extremely fine silk throughout the stone can actually enhance the value. Many rubies also display a strong red fluorescence to daylight, and this adds measurably to the beauty of this gem.
While a certain amount of silk is necessary to create the star effect in star ruby, too much silk desaturates the color, making it appear grayish. This is not desirable.

In the market, rubies are found in a variety of shapes and cutting styles. Ovals are cushions are the most common, but rounds are also seen, as are other shapes, such as the heart or emerald cut. Slight premiums are paid for round stones, while slight discounts apply for pears and marquises. Stones that are overly deep or shallow should generally be avoided.
Cabochon-cut rubies are also common. This cut is used for star stones, or those not clean enough to facet. The best cabochons are reasonably transparent, with nice smooth domes and good symmetry. Avoid stones with too much excess weight below the girdle, unless they are priced accordingly.

With the exception of imperial jadeite and certain rare colors of diamond, ruby is the world’s most expensive gem. But like all gem materials, low-quality (i.e., non-gem quality) pieces may be available for a few dollars per carat. Such stones are generally not clean enough to facet. The highest price per carat ever paid for a ruby was set on April 12, 2005 for an 8.01-ct. faceted stone that sold for $274,656 per carat ($2.2 million) at Christie’s New York. Previously the record for per-carat price was Alan Caplan’s Ruby (‘Mogok Ruby’), a 15.97-ct. faceted stone that sold at Sotheby’s New York, Oct., 1988 for $3,630,000 ($227,301/ct).

Stone Sizes
Large rubies of quality are far more rare than large sapphires of equal quality. Indeed, any untreated ruby of quality above two carats is a rare stone; untreated rubies of fine quality above five carats are world-class pieces.

Ruby may display asterism, the star effect. Fine star rubies display sharp six-rayed stars well-centered in the middle of the cabochon. All legs of the star should be intact and smooth. Just having a good star does not make a stone valuable. The best pieces have sharp stars against an intense crimson body color. Lesser stones may have sharp stars, but the body color is too light or grayish. On occasion, 12-rayed star sapphires are found. Inexpensive star rubies come mainly from India.

4.86 ct. star ruby
This 4.86-ct. star ruby from Pala International is one of the finest examples to come out of Mogok in years. (Photo: John McLean; Gem: Pala International)

The name “ruby” is believed to be derived from the Latin ruber, a word for red. According to Oriental beliefs, ruby is the gem of the sun. It is also the birthstone of July.

The original locality for ruby was most likely Sri Lanka (Ceylon), but the classic source is the Mogok Stone Tract in upper Burma. Fine stones have also been found in Vietnam, along the Thai/Cambodian border, in Kenya, Tanzania, Afghanistan, Pakistan, Yunnan (China) and most recently, Madagascar. Low-quality rubies also come from India and North Carolina (USA).

Today, the vast majority of rubies are heat-treated to improve their appearance. The resulting stones are completely stable in color. Many rubies are also heated in the presence of a flux to heal their fractures, particularly those from Möng Hsu, Burma. In lower qualities and smaller sizes, heat treated stones sell for roughly the same as untreated stones of the same quality. However, for finer qualities, untreated stones fetch a premium that is sometimes 50% or more when compared with treated stones of similar quality. Other treatments, such as oiling, dying and surface diffusion are seen on occasion. As with all precious stones, it is a good practice to have major purchases tested by a reputable gem lab, such as the GIA or AGTA, to determine if a gem is enhanced.

Synthetic rubies have been produced by the Verneuil process since the 1890s and cost just pennies per carat. Ruby has also been produced by the flux, hydrothermal, floating zone and Czochralski processes. Doublets consisting of natural sapphire crowns and synthetic ruby pavilions are fairly common, particularly in mining areas. Synthetics are also common at the mines, in both rough and cut forms.

Properties of Ruby

  Ruby (a variety of corundum)
Composition Al2O3
Hardness (Mohs) 9
Specific Gravity 4.00
Refractive Index 1.762–1.770 (0.008) Uniaxial negative
Crystal System Hexagonal (trigonal)
Colors Various shades of red.
Ruby is colored by the same Cr+3 ion that gives alexandrite and emerald their rich hues.
Pleochroism Strongly dichroic: purplish red/orangy red
Phenomena 6 or 12-rayed star
Handling No special care needed
Enhancements Frequently heated; frequently flux-healed; occasionally oiling, dying, surface diffusion
Synthetic available? Yes

Further Reading

The Collector Gem Buying Guides

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