Did You Know?
Ruby – from the Latin word ruber, which means red – is the most valuable variety of the corundum mineral species, and can command the highest per-carat price when it comes to coloured stones. Chromium is the trace element that makes a ruby red, which ranges from an orange-red to a purplish red.
History and Lore
In Sanskrit, the ruby is called ratnaraj, or the ‘king of precious stones’. The bright gem can be traced back to the 1st century AD where the Roman scholar Pliny described the hardness and density of ruby in his encyclopaedic work titled ‘Natural History’. Ancient Hindus believed that those who offered fine rubies to Lord Krishna were granted rebirth as emperors.
The 4Cs: Clarity, Colour, Cut and Carat Weight
Ruby hues range from orange-red to purplish red. The finest ruby has a pure, vibrant red to a slightly purplish red hue. As the hue becomes orange or more purplish, the ruby moves down the quality scale into good and commercial ranges.
Medium to medium-dark tones are preferred as long as the tone isn’t dark enough to have a negative effect on brilliance. At the other extreme, if the tone is too light, the stone is considered to be a pink sapphire.
Nearly all rubies have inclusions; some inclusions can contribute positively to a ruby’s appearance. The presence of rutile silk causes light to scatter across facets that might otherwise be too dark. This adds softness to the colour and allows the colour to spread more evenly across the ruby’s crown.
If large and prominent inclusions are located under the table facet, they can greatly diminish the transparency, brilliance and value of the stone. Inclusions can also limit a ruby’s durability.
The most common shapes of fashioned rubies are ovals and cushions with brilliant-cut crowns and step-cut pavilions. Round, trillion, emerald-cut, pear and marquise rubies are also available, but these shapes are rare in larger sizes and higher qualities.
Fine-quality rubies over one carat are rare, but commercial-quality rubies are commonly available in sizes from 0.15 carat and up.
Almost all rubies today are heat-treated to improve colour or clarity. These modifications can dramatically affect a ruby’s appearance and value. Large, top-quality untreated rubies can command prices much higher than their treated counterparts because they are rare.
It’s not always possible to detect heat treatment without lab testing, but you should assume that a ruby is heat-treated unless you see conclusive evidence that it’s not. An experienced gemmologist can usually detect heat treatment by examining a ruby’s inclusions under magnification.
While the trade generally accepts heating, other ruby treatments – including dyeing with coloured oils or filling surface fissures with epoxies or glasses – aren’t as well received. Today, lead-glass filled rubies are commonly found in the market, but they have durability problems and can be damaged by many jewellery repair procedures or exposure to common household chemicals, such as some cleaning solvents. These treatments, which often go undisclosed to buyers, are not permanent, and require special care.
Some corundum is also subjected to diffusion treatment – adding a colouring agent to the heating process – which may significantly improve a ruby’s colour. This treatment is permanent, but significantly affects the value of the stone because the colour is not natural.
How to Care for Rubies
Ruby-studded jewellery can be cleaned in an ultrasonic cleaner or steamed, but this should be done by a professional only after a close inspection to determine if any fillers or dyes are present.
As with most gemstones, a soft moistened cloth or a soft bristle toothbrush can be used with mild soap and water to clean the gem.