Sapphire, a variety of the mineral corundum, is prized globally for its brilliant color and exceptional durability. Composed of aluminum oxide, it ranks second only to diamond in hardness, making it a highly sought-after gemstone for jewelry. The high value of sapphires has led to a market filled with simulants, synthetics, and treated stones, making accurate identification important for buyers. Determining authenticity involves a systematic process of visual observation, physical testing, and microscopic examination of its internal characteristics.
Initial Visual Assessment and Color Characteristics
The first step in identifying a sapphire involves a careful examination of its surface and how it interacts with light. Sapphires typically exhibit a high luster, described as adamantine to vitreous, which results in a bright, reflective surface quality. When viewed under strong illumination, a well-cut sapphire will show a strong return of light, giving the stone a lively appearance that is often difficult to replicate in less refractive simulants such as glass.
Color is another factor for initial assessment, focusing on its distribution throughout the stone. Natural sapphires often exhibit color zoning, where the hue is unevenly distributed, appearing as lighter or darker bands that formed during crystal growth. A stone with a color that is too perfect or uniformly saturated, lacking any internal variation, may suggest artificial enhancement or a lab-created origin.
Pleochroism, the tendency of a crystal to show different colors when viewed from different angles, is a characteristic property of corundum. For example, a blue sapphire may appear violetish-blue in one direction and slightly greenish-blue when rotated. Observing this subtle shift in color by rocking the stone under a light source helps rule out materials like cubic zirconia or glass, which lack this optical property.
Utilizing Hardness to Rule Out Simulants
The most distinguishing physical property of a genuine sapphire is its superior hardness, a feature that immediately separates it from most common imitations. The Mohs scale of mineral hardness, which ranks minerals based on their scratch resistance, places sapphire at a remarkable 9. This means that only a diamond, which is rated at 10, is capable of scratching a sapphire.
This high rating is significant because many common simulants are substantially softer. Quartz, which is ubiquitous in nature, has a hardness of 7, while materials like glass are rated around 5.5. Even other popular gemstones used as sapphire alternatives, such as tanzanite, only reach a hardness of 6.5 to 7.
In a practical application, a true sapphire should not show surface wear or scratches from everyday exposure to dust, which often contains quartz particles. Using a scratch test on a valuable, mounted stone is inadvisable due to the risk of damage. Instead, assess the stone’s existing facet edges for signs of abrasion that would indicate a lower hardness. The exceptional durability of corundum contributes to the sharp, well-maintained facet edges seen on genuine sapphires, contrasting with the rounded edges often present on softer imitations.
Internal Evidence: Inclusions, Treatments, and Synthetics
A microscopic examination, ideally with at least a 10x magnification jeweler’s loupe, provides the most detailed information about a sapphire’s origin and history. This internal evidence helps distinguish between a natural stone, a synthetic stone, and a natural stone that has been treated for color or clarity enhancement. A sapphire that appears entirely flawless under magnification should be viewed with suspicion, as perfectly clean natural stones are exceedingly rare.
Natural sapphires typically contain characteristic inclusions that formed during their geological growth. One of the most common is “silk,” which refers to fine, needle-like crystals of rutile that can give the stone a desirable velvety appearance. Other natural indicators include small mineral crystals and “fingerprint” inclusions, which are intricate, partially-healed fracture planes within the crystal structure.
Synthetic sapphires, grown in a laboratory using methods like the Verneuil or Czochralski processes, exhibit different internal markers due to their rapid, controlled growth. Sapphires created using the flame-fusion (Verneuil) method often show curved growth striae, which are curved color bands visible under magnification, a feature never seen in natural corundum. They may also contain tiny, spherical gas bubbles trapped within the material.
Identifying common treatments is another important aspect of internal analysis. Heat treatment, which is widely accepted but affects value, is used to intensify color and improve clarity. Signs of heat treatment include the alteration of natural inclusions, such as the partial dissolution of rutile silk or the presence of melted crystal inclusions resembling small, white “snowballs.” More aggressive treatments, like fracture filling, may leave traces of glassy or flux residue in surface-reaching fissures.