Step 5: Advanced Gem Information

Lesson 3

What Are Phenomenal Gems?

Some optical effects in gemstones, such as color, brilliance, dispersion, and birefringence, can be quite impressive. However, not all optical effects are considered phenomenal. A few gems do things with light that may seem magical. These are called phenomenal gems. Some of the most well-known examples in this category are opals with their fabulous play of color, alexandrites that change color, and star sapphires. This article covers these and more phenomenal effects as well as how to grade gemstones that display them.

alexandrite - phenomenal gems

Phenomenal Gems: Key Terms

  • Interference occurs when light reflecting from below the surface of a gem overlaps light reflecting from higher up. (This effect can be seen on soap bubbles and oil slicks, too).
  • Scattering means “to strew about.” Some phenomenal effects are the result of scattering, but the term is not used alone in gemology.
  • Sheen is light reflected off a shiny surface. All polished gemstones have sheen, but this has special relevance in chatoyant and adularescent stones.

Valuing Phenomenal Gems

In most cases, these rare phenomena add beauty as well as value to a gem. Very common or very…