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It was the highest price ever paid at an auction for a single sapphire. While it might seem so simple a thing, the coloration process that makes a sapphire a sapphire is rarely a perfect procedure. If it goes right, however, the most beautiful bluish hues are produced, and the sapphire will instantly be a sought-after commodity. Home Gemstones. March 22, Blue Sapphire Read on to learn more about sapphires, how they form, and what process is required to change the color of a sapphire.
Most of the stones were recovered, including the Star of India, of course. Tags: Sapphire. This charge-transfer process between the ferrous and ferric forms of iron , present as impurities in different solid environments, is a ready source of color.
It makes glass bottles brown, it makes sandstone brown often seen in brownstone buildings , and it is the source of the blue in the pigment Prussian Blue Fe 4 Fe CN 6. Blue aquamarine. Beryls containing chromium impurities appear green and are known as emeralds. This is attributed to ligand field effects. However, the presence of iron impurities in beryl produces the color of aquamarines. The mechanism of charge transfer between two different ionic forms of iron give aquamarine its characteristic greenish-blue color.
Charge transfer is also responsible for the colors of a number of pigments. Substances that display their colors even when ground very finely are good candidates for pigments. The charge transfer process occurs both in whole crystals and their powdered forms, making these substances ideal pigments. In this case, the charge transfer process occurs between metal and ligand atoms. An example is the oxygen-to-chromium charge transfer in yellow chromate K 2 CrO 4.
The chromium cation will have a strong attraction for electrons from the oxygen anions, allowing a ligand-to-metal anion to cation electron transfer.
This mechanism is also present in sapphires, but absorption occurs only in the ultraviolet range, and does not affect the color we perceive. It turns out that many colored pigments , including red and yellow ochre that occur naturally used by the cave artists of the Ice Age, by Michelangelo on the Sistine Chapel ceiling, and by artists today , achieve their characteristic colors through the process of charge transfer.
Ultramarine left achieves its brilliant color through the process of charge transfer. Ultramarine was used by the ancient Egyptians, the French Impressionists, and is still in use today. This is an example of anion-anion charge transfer, where the color stems from charge transfer between the three sulfur anions.
Pleochroism refers to different bodycolors in different crystal viewing directions. Blue sapphires often have greenish blue and violetish blue pleochroism. Star corundum must be cut as a cabochon to display asterism. The cabochon must have an appealing appearance, with the star properly centered when the gem rests on its base. 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.
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. 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. Sapphire Quality Factors. Madagascar is a relative newcomer as a major sapphire source, but its stones can have an intense blue color that rivals the finest from more traditional sources.
Some pink sapphires have intensely saturated color. These sapphires range from yellow to a deep yellow-orange color called golden to highly saturated reddish orange. Courtesy: Varujan Arslanyan. 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. 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. Tiny inclusions create magnificent reflections of light known as the star effect. 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.
Courtesy: Lewis Allen. These sapphires are miniature works of art that have been expertly cut into custom designs. Courtesy: Mark Gronlund, Umatilla, Florida. Besides coming in all colors, corundum also comes in all cutting styles. Courtesy: Ballerina Gems. The incredible blue sapphire in the center of this ring weighs Sapphires are available in all sizes, shapes, and colors.
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