Sapphires rarely exhibit the high clarity of fine diamonds. Sapphires are host to many different inclusions, and even the best stones are not expected to be free of inclusions when viewed at 10x magnification. In fact, a sapphire with no inclusions should be viewed with suspicion; it may be a synthetic stone or a glass imitation.
The best clarity grade for sapphires is “eye-clean,” which means no inclusions are visible to the naked eye. When evaluating the clarity, experts consider the size, number, location, and overall visibility of the inclusions.
Inclusions typically found in sapphires include the following items:
Cavities – voids or holes that extend from the surface of a gemstone into its interior.
Color Zoning – areas or bands of alternating color in a gemstone.
Concaves/Naturals – natural indentations in the surface of a gemstone created during crystal formation. These growth marks are usually found on the girdle and do not affect the gemstone’s beauty or luster.
Feathers – small inclusions that have the wispy appearance of a feather.
Fingerprint Inclusions – networks of tiny liquid-filled tubes that resemble human fingerprints. Fingerprint inclusions are formed when sapphires re-crystallize to partially heal a fracture zone.
Halo or Discoid Fractures – oval or circular “fried egg” fractures surrounding a solid or liquid inclusion. These are formed from stress due to the radioactive decay of tiny zircon crystals or from the high temperatures required for heat treatment.
Included Crystals – light, dark, transparent or opaque minerals trapped inside a gemstone. In sapphires, you can frequently find minute crystals of hematite, zircon, spinel, calcite, and mica, for example.
Liquid Inclusions – liquid-filled spaces within a gemstone.
Silk – the fine rutile needles that create the sleepy transparency of some sapphires. Silk is also responsible for asterism in star sapphires. Not all sapphires contain silk. Silk is an example of an inclusion that may actually add value to a sapphire. In moderate amounts, the highly reflective rutile needles scatter light within a cut sapphire, helping to illuminate a stone’s darker facets and enhance its brilliance.
The condition of a sapphire’s silk is also a valuable clue to the stone’s treatment history. Many sapphires are heated to alter their color or enhance their clarity. The intense heat applied to treat sapphires partially melts or decomposes silk. Intact silk is strong evidence that a sapphire has not been heat treated, while degraded silk, recognizable under magnification by a trained gemologist, indicates that a sapphire has been heated.