Saturday, September 19, 2015

Star Stones: The Allure of Asterism

This spectacular vintage star sapphire cocktail ring is centered with a massive 29.5-carat natural blue star sapphire. Cut en cabochon, this gorgeous stone has a complete, strong, 6-ray star that is slightly off center with excellent movement. Surrounding the beautiful center stone is a double halo of sapphires and diamonds. The inner circle is set with dark blue round faceted sapphires, while the outer circle is set with round brilliant white diamonds.

Asterism is a remarkable phenomenon in which a cabochon-cut gemstone reflects a 4- or 6-rayed star radiating out from the focal center of the stone. Asterism is most frequently seen in corundum, among members of the sapphire family. However, it is also found in varieties of quartz, moonstone, and garnet.{1}

Asterism occurs naturally when a stone contains rutile needle inclusions called silk. These intrusions in the crystal structure must occur in a specific pattern in order to reflect the star-like pattern on the polished surface of the stone.{2}

The most prized star sapphires have a star pattern which appears centered on the stone with long-reaching, crystal-clear rays.{3} In order to achieve this desired effect, skilled cutters must often cut a lower dome, leaving slightly bulging sides. In addition, they often leave more depth at the base of the stone than they might for other cabochon-cut stones. The bulging sides ensure better centering of the asterism, while the extra depth provides greater clarity of the star effect.{4} While these unusual parameters make it a little tricky for jewelers to mount a star sapphire, the results are worth far more than the little bit of trouble they might encounter.

When choosing a star gemstone, we recommend working closely with a reputable and knowledgeable jeweler. Asterism is a rare phenomenon, commanding a higher price per carat, and therefore requires specialized knowledge. Ask your jeweler to show you their selection under bright, direct light, since under diffused light the stone will not perform optimally.

Star stones are best examined with a single source of light, which allows you to move the light back and forth across the surface. This will demonstrate that the rays of the star are straight, nearly centered, and distributed evenly across the surface of the stone. Also, remember to include color, cut, and clarity in your assessment of a star stone.

The star will move across the surface of the stone, but it should appear somewhat static in its size, direction, and orientation. The highest quality star stones exhibit sharp and unwavering rays which stretch from girdle to girdle.{5}

At EraGem, we have a select number of star sapphires available. They range in color from light and bright pink (rubies) to pale and deep blue. It would be our pleasure to visit with you in our Seattle-area showroom to demonstrate the mesmerizing beauty of these phenomenal gemstones. Call today to make your appointment with one of our knowledgeable associates.

~Angela Magnotti Andrews, Staff Writer


  1. Smigel, Barbara W., "Star Stones (Asterism)," accessed at on August 15, 2015.
  2., "About Star Gemstones," accessed August 15, 2015.
  3. The Star Ruby Shop, "Cutting Process of Asteriated Gemstones," accessed from on August 15, 2015.
  4. Smigel.
  5. Waters, Michelle, "Gemstones with Asterisms and Chatoyancy," Harriet Kelsal Jewelry Website, accessed on August 15, 2015.

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