Sapphire Quality Factors
Sapphires come in a wide range of colours, and each colour has its own quality variations. In general, the more intense the colour and the fewer the distracting zones of unattractive colour, the more valuable the stone.
Colour has the most important influence on blue sapphire’s value. The most highly valued blue sapphires are velvety blue to violetish blue, in medium to medium dark tones. Preferred sapphires also have strong to vivid colour saturation. The saturation should be as strong as possible without darkening the colour and compromising brightness. Sapphires with these qualities command the highest prices per carat.
At the other end of the price scale are commercial-grade sapphires with greenish blue colour or strong greenish blue pleochroism. Pleochroism is different colours seen in different crystal directions. Less valuable blue sapphires might be greyish, too light or too dark.
The major fancy sapphire colour categories are padparadscha, pink and purple, orange and yellow, green, and colourless and black. Each category has its own colour range, causes of colour and market.
The fancy sapphires that people in the trade call padparadscha are very beautiful. They typically have a high per-carat value, too—much higher than other types of fancy sapphires.
Their colour can be hard to describe. Some people say padparadscha sapphire colours should be called salmon or sunset. Others compare the colour to the flesh of a ripe guava.
In spite of these differing colour descriptions, people in the industry usually agree that padparadscha sapphire colours are intensely saturated and range from light to medium pinkish orange to orange-pink.
Pink sapphires range from red to purple with weak to vivid colour saturation and lighter tone. Purple sapphires are similar in colour but darker and always have purple as the dominant colour. They range from medium to dark reddish purple to violetish purple with weak to vivid colour saturation.
Corundum appears in an array of yellow and orange hues that includes bright lemon, soft peach and vivid tangerine.
In specific colour terms, yellow sapphires range from light to dark greenish yellow to orangey yellow with weak to intense colour saturation. The finest yellow sapphire is yellow to orangey yellow with vivid saturation.
Orange sapphires range from yellowish orange to reddish orange. The finest orange sapphires are strong, pure orange to red-orange with medium tone and vivid saturation.
Green sapphires range from light to dark bluish green through to yellowish green, and are usually low in saturation. Green sapphire is readily available, but its colour isn’t very marketable. Its colour is sometimes described as khaki or olive. That’s because the stones tend to have low saturation or unattractive colour zoning.
People in the trade refer to corundum in its purest form as either colourless sapphire or white sapphire. The closer corundum comes to having no colour, the more valuable it is as a colourless sapphire. Traces of extremely light grey, yellow, brown and blue are common, and reduce the value. Colourless sapphires have been popular as small accent stones in jewellery.
Colour-change sapphires are corundum’s chameleons—stones that change colour under different lighting. Under daylight equivalent (fluorescent) light, the typical colour-change sapphire’s basic colour ranges from blue to violet. Under incandescent light, it ranges from violetish purple to strongly reddish purple. Some rare colour-change sapphires change from green in daylight to reddish brown in incandescent light.
When gem experts judge colour-change sapphires, they describe the colour change as weak, moderate or strong. The strength of the stone’s colour change is the most important quality factor affecting its value, followed in importance by the actual colours of the stone.
As with all coloured stones, the colour of star corundum has a great effect on its value. The best star corundum has a crisp, distinct star against strongly saturated colour. If the colour is too light, it doesn’t provide enough contrast for the star’s rays, and the star will be less visible.
Star corundum can be red, blue, black, grey, purple or yellow—practically every colour under the sun. The term “star sapphire” encompasses all colours of star corundum except red, which is called star ruby.
Naturally, some colours of star corundum are valued more highly than others. In general, the most prized colours are the same as the colours most valued in non-phenomenal corundum: red and blue.
Trade terms based on sources 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 that from classical sources or with a slightly different, but just as beautiful, appearance.
Blue sapphires typically have some inclusions, but they generally have better clarity than rubies. Blue sapphires with extremely high clarity are rare, and very valuable.
Several types of inclusions are found in sapphires. Among these are long thin mineral inclusions called needles. Fine needles are called silk when they occur as the mineral rutile in intersecting groups. Other clarity characteristics in sapphire are included mineral crystals, partially healed breaks that look like fingerprints, colour zoning and colour banding.
Generally, inclusions make a stone less valuable. The price can drop substantially if the inclusions threaten the stone’s durability. Even so, inclusions can actually increase the value of some sapphires. Many of the most valuable Kashmir sapphires contain tiny inclusions that give them a velvety appearance. They scatter light, causing the coveted visual effect without negatively affecting the gem’s transparency.
Star rubies and star sapphires belong to the phenomenal corundum category. The star effect is called asterism. It’s caused by reflections from tiny, needle-like inclusions that are oriented in several specific directions. Stars are usually made up of 2, 3 or 6 intersecting bands, resulting in 4, 6 or 12 rays.The most common stars have 6 rays, and 12-rayed stars are quite rare. Two different sets of inclusions—one of rutile and one of hematite—oriented in slightly different directions can cause a 12-rayed star.
Hematite inclusions cause asterism in black star sapphires. The sapphire’s colour is actually yellow, green or blue, but the inclusions make it appear dark brown or black.
The finest star is distinct, centred on the top of the stone and visible from a reasonable distance, about arm’s length. The star’s quality should be the same when viewed from all directions.
The rays should be uniform in strength, reach from one side to the other and intersect at the top of the stone. They should be straight, not fuzzy, wavy or broken, and they should contrast strongly with the background colour. The star should also have elegant “movement”. This means that, as you rock the stone, the star should move smoothly across the surface with no dead spots.
The best and most expensive star corundum is semi-transparent, with just enough silk to create a well-defined star. Too much silk can harm transparency and also lead to poor colour, lowering the value of the stone considerably.
The shape of a rough sapphire crystal influences the finished stone’s shape and size. Rough sapphire’s most common crystal form is a barrel- or spindle-shaped hexagonal pyramid. For this reason, finished sapphires are often deep.
To achieve the best overall colour, maintain the best proportions and retain the most weight possible, cutters focus on factors like colour zoning, pleochroism, and the lightness or darkness of a stone.
Colour zoning—areas of different colours in a stone—is a common sapphire characteristic. Blue sapphire often has angular zones of blue and lighter blue. To accommodate colour zoning in some sapphires, cutters orient the concentrated colour in a location that offers the best visible colour in the cut stone.
In Sri Lankan sapphires, the colour is often concentrated close to the surface of the crystal. If a cutter can orient the culet within the concentrated area of colour, the stone will appear entirely blue in the face-up position.
Pleochroism is different colours in different crystal directions. Blue sapphires often have greenish blue and violetish blue pleochroism. It’s most desirable to orient the cut so that the stone shows the violetish blue colour when it is set in jewellery.
Star corundum must be cut as a cabochon to display asterism. A finished stone’s attractiveness depends on the star’s orientation and the cabochon’s symmetry, proportions and finish.
The cabochon must have an appealing appearance, with the star properly centred when the gem rests on its base. The stone’s outline should be symmetrical.
For most stones, the dome should be fairly high—about two-thirds of the stone’s width—to focus the star sharply. If it’s too high, the phenomenon loses its graceful motion when the stone is tilted. Excessive height also makes the stone difficult to mount.
If the dome is cut too shallow, the star will only be visible from directly above. Black star sapphires, however, are prone to parallel breaks, so they’re usually cut very flat to reduce the risk of damage.
A stone should not have excess weight below the girdle that doesn’t contribute to the optical effect or reinforce colour.
Blue sapphires can range in size anywhere from a few points to hundreds of carats, and large blue sapphires are more readily available than large rubies. However, most commercial-quality blue sapphires weigh less than 5.00 carats.
Large commercial-quality blue sapphires are more common than large fine-quality ones. As a result, size makes more of a difference in the price of fine-quality sapphire. A fine-quality 5.00 carat blue sapphire sells for approximately five times more per carat than the same-quality 1.00 carat stone.
A commercial-quality 5.00 carat stone sells for only about twice as much per carat as a 1.00 carat blue sapphire of the same quality.
These examples are not meant to be exact pricing guidelines, but to illustrate how much the per-carat price can go up as the size and the quality rise.