Sapphires come in a wide range of colors, and each color has its own quality variations. In general, the more intense the color and the fewer the distracting zones of unattractive color, the more valuable the stone.
Color 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 color saturation. The saturation should be as strong as possible without darkening the color 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 color or strong greenish blue pleochroism. Pleochroism is different colors seen in different crystal directions. Less valuable blue sapphires might be grayish, too light, or too dark.
The major fancy sapphire color categories are padparadscha, pink and purple, orange and yellow, green, and colorless and black. Each category has its own color range, causes of color, 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 color can be hard to describe. Some people say padparadscha sapphire colors should be called salmon or sunset. Others compare the color to the flesh of a ripe guava.
In spite of these differing color descriptions, people in the industry usually agree that padparadscha sapphire colors 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 color saturation and lighter tone. Purple sapphires are similar in color but darker and always have purple as the dominant color. They range from medium to dark reddish purple to violetish purple with weak to vivid color saturation.
Corundum appears in an array of yellow and orange hues that includes bright lemon, soft peach, and vivid tangerine.
In specific color terms, yellow sapphires range from light to dark greenish yellow to orangy yellow with weak to intense color saturation. The finest yellow sapphire is yellow to orangy 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 yellowish green, and are usually low in saturation. Green sapphire is readily available, but its color isn’t very marketable. Its color is sometimes described as khaki or olive. That’s because the stones tend to have low saturation or unattractive color zoning.
People in the trade refer to corundum in its purest form as either colorless sapphire or white sapphire. The closer corundum comes to having no color, the more valuable it is as a colorless sapphire. Traces of extremely light gray, yellow, brown, and blue are common, and reduce the value. Colorless sapphires have been popular as small accent stones in jewelry.
Color-change sapphires are corundum’s chameleons—stones that change color under different lighting. Under daylight equivalent (fluorescent) light, the typical color-change sapphire’s basic color ranges from blue to violet. Under incandescent light, it ranges from violetish purple to strongly reddish purple. Some rare color-change sapphires change from green in daylight to reddish brown in incandescent light.
When gem experts judge color-change sapphires, they describe the color change as weak, moderate, or strong. The strength of the stone’s color change is the most important quality factor affecting its value, followed in importance by the actual colors of the stone.
As with all colored stones, the color of star corundum has a great effect on its value. The best star corundum has a crisp, distinct star against strongly saturated color. If the color 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, gray, purple, or yellow—practically every color under the sun. The term “star sapphire” encompasses all colors of star corundum except red, which is called star ruby.
Naturally, some colors of star corundum are valued more highly than others. In general, the most prized colors are the same as the colors most valued in non-phenomenal corundum: red and blue.
Trade terms based on sources can represent certain colors 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 color 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, color zoning, and color banding.
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 color is actually yellow, green, or blue, but the inclusions make it appear dark brown or black.
The finest star is distinct, centered 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 against the background color. 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 color, 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 color, maintain the best proportions, and retain the most weight possible, cutters focus on factors like color zoning, pleochroism, and the lightness or darkness of a stone.
Color zoning—areas of different colors in a stone—is a common sapphire characteristic. Blue sapphire often has angular zones of blue and lighter blue. To accommodate color zoning in some sapphires, cutters orient the concentrated color in a location that offers the best visible color in the cut stone.
In Sri Lankan sapphires, the color is often concentrated close to the surface of the crystal. If a cutter can orient the culet within the concentrated area of color, the stone will appear entirely blue in the face-up position.
Pleochroism is different colors in different crystal directions. Blue sapphires often have greenish blue and violetish blue pleochroism. It’s most desirable to orient the cut so the stone shows the violetish blue color when it is set in jewelry.
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 centered 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 be visible only 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 color.
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.