Insights From Inclusions

Natural gemstones are typically far more valuable than synthetics, so being able to identify them correctly is a powerful skill. One of the best ways to determine if a gemstone is natural or synthetic is to note the type and variety of its inclusions.

Certain types of inclusions are found more often in natural gemstones than in their synthetic counterparts. Needles, clouds, fluids, and crystals are examples of inclusions typically found in natural gemstones.

Intersecting Rutile Needles
Intersecting rutile needles (called “silk”) are inclusions commonly found in corundum. Photo by John I. Koivula/GIA
Deep Red Needles of Hematite in Amethyst
A common inclusion observed in amethyst is deep red needles of hematite (magnified 10×). Hematite needles are thin, elongated inclusions. They appear only in natural gemstones. Photo by John I. Koivula/GIA

Synthetic gems generally have inclusions specific to their method of growth. These characteristics include translucent to opaque coloring, coarseness, and a web-like appearance, which are stark contrasts to natural, transparent, partially healed fracture “fingerprint” inclusions. For example, translucent to opaque-white to yellowish granular-looking flux are types of inclusions you would typically find in a flux-grown synthetic stone.

Primary Flux Inclusions
Hallmark inclusions of a flux-grown Kashan synthetic ruby are translucent to opaque-white primary flux. Photo by Nathan Renfro/GIA

“By understanding the types of inclusions and their characteristics, you can determine if a gem is natural or synthetic. That helps you understand the stone’s history and value,” says Brenda Harwick, GIA’s manager of on-campus and laboratory gemology instruction in Carlsbad.

The three-volume Photoatlas of Inclusions in Gemstones, authored by Eduard Josef Gübelin and John Koivula, is widely considered a definitive read on the subject. To learn more about these valuable resources visit and enter keyword “Koivula.”

Some GIA students become so curious about the particular types of minerals that make up inclusions that they pay less attention to the general type of inclusion they are examining.

Here’s a helpful study tip: First become adept at identifying the overall types of inclusions, and then pursue your natural curiosity as to their specific make-up. Advanced chemical and structural testing is often required to determine what they are, but rutile and zircon are some common culprits (depending on the species of gemstones).

Remember, though, it’s the type and number of inclusions that really matter – and not their precise chemical composition. Keep this in mind, and you will become a better gemologist.

Montana Sapphire with Dark Rutile Crystals
This Montana sapphire contains dark rutile crystals. When the stone is heat-treated, the titanium dissolves into the host sapphire combining with iron to enhance the blue color of the stone. This internal diffusion of blue color is proof of heat treatment in sapphire. Photo by John I. Koivula/GIA
Hydrothermal Growth in a Synthetic Aquamarine
This is hydrothermal growth in a synthetic aquamarine. This chevron type of growth structure is indicative of the synthetic origin of this aquamarine. Photo by Nathan Renfro/GIA

Tips for Viewing Inclusions

By looking at gemstones under a microscope, you’ll see the number and general type of inclusions. Doing this can help you determine if a gem is natural or synthetic; and that information can affect a gemstone’s value.

Studying gems under a microscope also helps you create your own “visual library” of inclusions, which over time, will become references as you examine new stones.

Here are a few tips for using a microscope for this purpose.

  • Use a variety of lighting environments to examine inclusions, including: brightfield, darkfield, polarized light, and fiber optic illumination. This lets you see how the gemstone reacts to a controlled pathway of light, revealing valuable information about the inclusions. Each type of light may yield new information and help you see something you would otherwise miss if you only used one lighting environment.
  • First look through the broadest window into the gem, which is generally the table facet, and then into the gemstone’s pavilion (this often gives you an excellent view of the inclusions).
  • Be certain to rock and tilt the gemstone so that you view it from a variety of angles.