Ruby Quality Factors
For better-quality material, slight differences in colour can make significant differences in value. For top-colour ruby that’s also free of eye-visible inclusions, the price rises even more.
The per-carat price of ruby can also increase dramatically as size increases, especially for better-quality stones.
Colour is the most significant factor affecting a ruby’s value. The finest ruby has a pure, vibrant red to slightly purplish red colour. As the colour becomes too orangey or more purplish, the ruby moves down the quality scale. The highest-quality rubies have vivid colour saturation.
The colour must be neither too dark nor too light to be considered finest quality. If the colour is too dark it has a negative effect on the stone’s brightness. At the other extreme, if the colour is too light, the stone is considered pink sapphire, even if colour strength or intensity is high.
Some gem dealers debate the borderline between ruby and pink sapphire. Historically, the word ruby referred to shades of red, which technically included pink. There are also cultural differences in the interpretation of ruby versus pink sapphire. In some gem-producing nations such as Sri Lanka, pink colours have always been considered ruby, while in many consuming countries they are classified as pink sapphire.
The GIA Laboratory uses a controlled set of comparison stones called masterstones to determine if corundum is ruby or if it’s pink, purple or orange sapphire. The laboratory grades its masterstones on the principle that red must be the dominant hue before a stone can be called a ruby. In the gem trade, though, identification of the dominant hue is subject to personal perception.
Blood is another symbol of ruby’s colour. Descriptions have compared ruby to the “blood from the right ventricle” or the first two drops of blood from a freshly killed pigeon. Historically, the term “pigeon’s blood” described the red to slightly purplish or pinkish red colour of rubies with a soft, glowing red fluorescence.
Traditional descriptions like these are useful for evoking images and describing colour among professionals. But they can be subject to misinterpretation when used to describe a ruby’s actual colour.
Trade terms can represent certain colours and qualities that are associated with a stone’s source. The qualities might be typical of that source or they might represent the finest stones from that source.
But a single source never consistently yields gems that are all the same colour and quality. In fact, the descriptive term might represent only a small percentage of stones from that source. The appearance of stones from a particular source often varies over time, and the original quality associated with that source might no longer match the material produced.
New sources can produce material very similar to rubies from classical sources or with a slightly different appearance, but just as beautiful.
People in the trade expect rubies to have at least some inclusions because inclusion-free rubies are practically non-existent. Ruby value depends on how visible the inclusions are. Obvious inclusions or inclusions that reduce transparency or brightness lower a ruby’s value dramatically.
If large and prominent inclusions are located under the table facet, they greatly diminish the transparency, brilliance and value of the stone. Inclusions can also limit a ruby’s durability. Significant surface-reaching fractures can pose durability threats.
Typical ruby clarity characteristics include thin mineral inclusions called needles. When the mineral is rutile and needles are present in intersecting groups, it is called silk. Needles might be short or long and slender, and they might appear to be woven tightly together.
Ruby can also contain needles composed of other minerals, small crystals, zones of colour variation or inclusions that resemble fingerprints.
Some inclusions can actually contribute positively to a gem’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 spreads the colour more evenly across the ruby’s crown.
Needles that intersect can also cause the star effect, called asterism, when the stone is cut with a curved upper surface.
Several factors affect the cut and proportion of rubies on the market. A ruby’s crystal shape dictates its suitability for certain cuts. The most common shape is a flat tabular hexagonal shape, but ruby crystals from some sources can be elongated.
To accommodate these crystal shapes, the most common shapes of fashioned rubies are 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. But these shapes are rare in larger sizes and higher qualities.
Ruby rough is very expensive, so cutters try to conserve as much weight as possible. They might fashion flattened ruby rough into shallow stones, even though light escapes through flattened pavilions, causing an unattractive see-through area in the stone called a window.
Pleochroism—the appearance of different colours 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 minimise the orangey red colour by orienting the table facet perpendicular to the long crystal direction. Even so, it’s not always possible to orient a ruby for ideal colour return because the potential loss of weight would be too great.
Size and weight
Fine-quality rubies over one carat are very rare, but commercial-quality rubies are commonly available in a wide range of sizes. The price per carat goes up significantly for ruby as it increases in size.
For example: A commercial-quality 5-carat ruby might sell for about twice as much per carat (10 times total stone value) as a commercial-quality 1-carat ruby, while a fine-quality 5-carat ruby sells for over five times more per carat (25 times total stone value) than a fine-quality 1-carat ruby.
These examples are not meant for exact pricing guidelines, but to illustrate how much the per-carat price can go up as the size and the quality rise.