cat's eye rubellite - phenomenal gemscat's eye rubellite - phenomenal gems

Phenomenal Gems: Cat’s Eyes, Star Stones, and More

Phenomenal gems seem to do magical things with light. Learn the science behind optical effects like cat’s eyes and star stones and how to judge their value.

18 Minute Read

HomeLearning CenterGemologyPhysical and Optical Properties of GemstonesPhenomenal Gems: Cat’s Eyes, Star Stones, and More

Phenomenal Gems: Key Terms

  • Interference occurs when light reflecting from below the surface of a gem overlaps light reflecting from above. (Soap bubbles and oil slicks display this effect, too).
  • Scattering means “to strew about.” Some phenomenal effects are the result of scattering.
  • Sheen is light reflected off a shiny surface. All polished gemstones have sheen, but this has special relevance in chatoyant and adularescent stones.

Grading Phenomenal Gems

In most cases, a phenomenal effect will add beauty as well as value to a gem. However, when evaluating phenomenal gems, gemologists must consider many factors. Exceptions can arise.


A gem’s relative rarity and popularity will often adjust the impact any phenomenal effect will have on its value.

For example, mother of pearl is a phenomenal gem that’s so common it’s not valuable. On the other hand, some stones are so rare there’s just little demand for them. Although very rare, tenebrescent hackmanite isn’t worth nearly as much as a more common, more popular star sapphire.

The Four Cs

Most of the grading criteria for phenomenal gems are the same as for other stones: color, clarity, carat and, to a lesser degree, cut. For example, star…

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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