Rubies are the most celebrated of red gemstones. Their name in Sanskrit means “the king of precious stones,” and the gems appear in the crown jewels of many countries. Even now, the stone can command the highest per carat prices of all colored gemstones. For many years, people believed that all crimson jewels were rubies. By the nineteenth century, scientists understood that rubies were a subset of the mineral corundum, and a close relative of sapphire. Rubies are made of two parts aluminum and three parts oxygen, with trace amounts of chromium providing their famous hues.
With a Mohs hardness of 9, rubies are durable enough for everyday wear, even on high traffic pieces like bracelets and rings. They have a trigonal lattice system, which influences how rubies appear in their rough state. Well-shaped rough resembles hexagonal barrels with a broad middle and tapered or flattened ends. It’s also common for rough to appear as flattened tablets.
Rubies may be either transparent, translucent or opaque. Stones that are transparent to translucent are usually faceted, while opaque jewels are more likely to be carved into beads or objects d’art. Multiple factors can determine ruby’s transparency, such as the type and quantity of internal characteristics, or the tone of the jewel.
The gem has a refractive index of 1.762 to 1.778, meaning the stone can slow and bend light to a greater extent than glass, though not as dramatically as a diamond. Under ultraviolet light, rubies fluoresce red, further highlighting their color. The luster, or surface shine, varies. They may appear dull or greasy in their rough state, and as bright as subadamantine when polished.
Depending on the source, rubies may be found among marble or basalt. Rubies with marble host rock have especially intense saturation. Basalt is richer in iron, and may impart this element to the precious stone. Iron in turn may decrease the fluorescence of rubies and give it a darker tone. Skilled cutters may give such jewels shallower pavilions to lighten them.
Ruby and Fancy Sapphire
Some rubies take on hints of other colors such as purple or orange. At times, their reds may be light in tone or saturation. The Gemological Institute of America places rubies in a hue range of strongly purplish red, slightly purplish red, red and orange red. Saturation, or color intensity, may run from low to vivid depending on quality factors like hue and tone.
Other countries and organizations may have different ideas about where rubies end and fancy sapphires begin. A pink sapphire in the United States might become a ruby in Sri Lanka. Rubies tend to have greater value than sapphires, which may lead some dealers to call a pinkish-red corundum a “ruby.” However, the United States has minimum color standards for rubies to reduce confusion within its borders.
Ruby’s high hardness rating makes it useful for industrial applications. Laboratory created rubies and other corundum are often used for abrasives such as sandpaper and emery. As it is transparent as well as durable, ruby and other corundum may be used for watch parts, scanning devices and even windows for spacecraft.
One of the more dramatic uses of industrial ruby is lasers. The first working laser was completed in 1960, using a one by one-and-a-half-centimeter rod of synthetic ruby. Due to the unusually long fluorescence time of rubies, the device was able to produce higher amounts of energy in longer pulses than other materials. Ruby lasers were used for a number of purposes, including medical treatments.
As with sapphire, ruby often contains thin needles of titanium dioxide, also known as rutile within its crystal. Rutile inclusions are referred to as “silk” and can provide a distinctive luster for rubies. Silk may also soften the jewel’s appearance, brighten its interior or create stars.
When rutile crystals lie parallel to one another, a band of light appears perpendicular to the needles. If there are multiple sets of parallel rutile silk stacked in different directions, rows of light intersect, creating asterism. Star rubies are among the most valued of phenomenal corundum, especially if it has a pure red body color.
Ruby deposits are found on multiple continents, most notably Africa and Asia. Of the former, most rubies are found on the eastern side of the continent, in countries like Madagascar, Malawi, Tanzania and Kenya. Madagascar is better known for sapphire mining, though the country has unearthed rubies of differing qualities. Other locations have large proportions of fine color rubies, but have not yet been mined to their greatest potential.
One of the most famous Asian sources for ruby is Myanmar. While the country produces jewels of varying appearance, the reddest, most intense hues are said to originate there. The country has been mining since the fifteenth century, and has supplied the majority of the world’s rubies since the 1990s.
Other Asian ruby sources of note are Thailand, Cambodia, Vietnam and Sri Lanka. Neighbors Thailand and Cambodia share a common deposit of basalt hosted gems. Vietnam’s jewels were discovered in the 1980s, among marble host rock. Their ruby colors there have been compared to those of Myanmar. Sri Lanka has been mining corundum for thousands of years and unearths silky, light toned rubies.
Fine rubies measuring more than 5 carat are very rare, though commercial quality rubies are available in a range of sizes. The larger the gemstone, the greater its value. When cutters shape the rough, they aim to preserve as much weight as possible. With flattened stones, this may manifest as wide, shallow jewels.
Rubies are commonly fashioned into ovals and cushion cuts. The upper halves of the stones are typically faceted in brilliant styles, while lower portions are step cut. Other cuts typical of rubies include round, pear, marquise, emerald cut, though these are more available in smaller carat sizes than larger.
Color is a profound factor in determining ruby’s quality. There are many poetic terms describing its pinnacle hue, including “sunrise,” “pomegranate” and “pigeon’s blood.” A clearer description of ideal color is pure red to a slightly purplish red. The tone should be medium to medium dark, with vivid saturation.
Rubies frequently possess clarity characteristics, though they don’t necessarily detract from the gemstone’s beauty. Rutile needles may lighten dark jewels or provide optical phenomena. Inclusions resembling fingerprints may also appear. Some stones may have color zoning, or uneven coloration in the crystal. Gem cutters do their best to orient the gem so its best hue is visible face up and the color appears uniform.