New sources can produce material that is very similar to gems from classical sources or with a slightly different, but just as beautiful, appearance.
Commercial-grade sapphires may contain a less desirable greenish blue color or strong greenish blue that is visible as you view the gem. Uniformly green sapphires that are saturated in color are actually rare and many collectors prize them. In green sapphires, a mix of yellow and blue sapphire accounts for the color a person sees.
Color Change Sapphires
Color-change sapphires are corundum’s chameleons—stones that change color under different lighting. Under daylight equivalent (fluorescent or LED daylight-balanced) 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.
The color of star corundum has a great effect on its value, though it is understood that miniscule inclusions are the cause of the asterism. They can affect transparency and color, and only very rare, exceptional gems exhibit transparency, depth of color and asterism. As such, 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, pink, blue, black, gray, brown, 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, generally they refer to 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 its production.
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.
These long, thin, intersecting inclusions of the mineral rutile are called silk. This arrangement can produce the star effect in corundum.
Generally, inclusions make a stone less valuable. 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 sapphires and star rubies 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 (rarely) 12 rays.
Tiny inclusions create magnificent reflections of light known as the star effect. - © GIA & Tino Hammid, courtesy Benjamin Zucker
Star sapphires usually have 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.
Ideally, the rays should be uniform in strength, reach from girdle to girdle on the cabochon, and intersect at the top center of the stone. The best stars are straight, not fuzzy, wavy, or broken. The reflective stars should contrast strongly against the gemstone’s bodycolor. The star should also have elegant “movement.” This means that, as you rock the stone, the star should appear to move smoothly across the surface.
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.
Oval shapes with triangular and kite-shaped facets on the gem's crown (top portion) and parallel rectangular facets on the gem's pavilion (bottom portion) are very common for corundum of all colors. Photo: Robert Weldon/GIA. Courtesy: Lewis Allen
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. 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 crystal to best determine how to orient the gem during cutting.
These sapphires are miniature works of art that have been expertly cut into custom designs. – Photo: Robert Weldon/GIA. Courtesy: Mark Gronlund, Umatilla, Florida.
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 refers to different bodycolors in different crystal viewing 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.
Besides coming in all colors, corundum also comes in all cutting styles. – Photo: Jeff Scovil. Courtesy: Ballerina Gems
Star Sapphire Cut
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.
SAPPHIRE CARAT WEIGHT
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.
Fine-quality blue sapphires in larger sizes are rare, but they’re still more available than ruby. The incredible blue sapphire in the center of this ring weighs 18.79 carats. - Vagabonde Bleue Ring © 2011 Fabergé Ltd
Large commercial-quality blue sapphires are rare, but more available than large fine-quality ones. As a result, size makes more of a difference in the price of fine-quality sapphire.
Sapphires are available in all sizes, shapes, and colors. Photo: Robert Weldon/GIA, Dr. Eduard J. Gübelin Collection.
This article is published by GIA ,,, original page link https://www.gia.edu/sapphire-quality-factor