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…