A Deep Dive Into Sapphire Inclusions

Sapphire inclusions are one of the most fascinating and informative aspects of these precious gemstones. Sapphires with excellent clarity are often valued the highest in the industry but there are times when inclusions can be greatly appreciated too, such as in star sapphires. 

What are sapphire inclusions? 

Inclusions are internal features formed during a sapphire’s natural growth. They play a critical role in determining a sapphire’s origin, authenticity, and overall character. Inclusions can act as a gemstone’s fingerprint, offering valuable insights into its geological history. 

How are inclusions formed? 

Inclusions in sapphires are formed under intense heat and pressure deep within the Earth’s crust. Sapphires belong to the mineral family corundum, composed primarily of aluminum oxide. As these crystals grow, they may trap tiny fragments of surrounding minerals, fluids, or gases. These trapped materials become inclusions, preserved within the sapphire as it continues to develop over millions of years. Because no two geological environments are identical, inclusions vary widely depending on where and how the sapphire formed.

What types of inclusions are there? 

RUTILE INCLUSIONS

One of the most well-known types of sapphire inclusions is rutile, a mineral composed of titanium dioxide. Rutile often appears as fine, needle-like structures within the stone, commonly referred to as “silk.” These delicate threads can intersect in various patterns, sometimes forming a dense network. When rutile silk is oriented in specific directions and the sapphire is cut as a cabochon, it can produce a phenomenon known as asterism. This effect appears as a star-like pattern on the surface of the stone, typically with six rays. Star sapphires are highly prized, and their value often depends on the sharpness and visibility of the star.

CRYSTAL INCLUSIONS 

Another common inclusion type are mineral crystals embedded within the sapphire. These can include minerals such as zircon & spinel. These crystal inclusions may appear as tiny, well-defined shapes, visible under magnification. 

FLUID INCLUSIONS

Fluid inclusions are also frequently found in sapphires. These inclusions contain tiny pockets of liquid, gas, or a combination of both, trapped during the crystal’s formation. Under magnification, fluid inclusions may appear as small bubbles, often arranged in patterns or clusters. The presence of fluid inclusions is an indicator that no heat treatment has been used to treat the stone. 

COLOUR ZONING 

Color zoning is another feature closely related to inclusions. While not always considered a traditional inclusion, it results from uneven distribution of trace elements during the sapphire’s growth. This can create bands or patches of varying color intensity within the stone. In blue sapphires, for example, iron and titanium are responsible for the color, and their uneven presence can lead to visible zoning. While some buyers prefer evenly colored stones, others appreciate color zoning as a sign of natural origin and uniqueness. Colour zoning is quite common in sapphire rough and can be fairly easily managed by effective cutting. 

CAVITY INCLUSIONS 

Negative crystals or cavities are a particularly intriguing type of inclusion. These are essentially voids within the sapphire that take on the shape of a crystal. Unlike solid mineral inclusions, negative crystals are empty or filled with fluid or gas. They often have sharp, angular outlines and can sometimes mimic the appearance of actual crystals. Their presence can provide further evidence of natural formation and can be useful in distinguishing natural sapphires from synthetic ones.

Do lab grown sapphires have inclusions? 

Lab-created sapphires may contain inclusions but they are often different in appearance from natural ones. For example, synthetic sapphires created using the Verneuil process may exhibit curved growth lines or gas bubbles, which are rarely seen in natural stones. 

Inclusions also play a crucial role in identifying synthetic sapphires. By studying inclusions under magnification, gemologists can often determine whether a sapphire is natural or man-made.

Are heated sapphires easy to spot? 

Yes, heated sapphires are easy to spot. While to the naked eye - a natural heated sapphire is indistinguishable from a natural unheated sapphire, under microscope, a gemologist can easily detect heat treatments. Heat treatments change the internal characteristics significantly. 

Advancements in gemological technology have made it easier than ever to study sapphire inclusions in detail. Tools such as microscopes, spectroscopy, and advanced imaging techniques allow experts to analyze inclusions with remarkable precision. 

Do inclusions influence sapphire value? 

Yes - in general, gemstones with fewer visible inclusions are considered more valuable, particularly in faceted stones where clarity is highly valued. Visible & or excessive inclusions in sapphire typically see lower prices. 

However, this is not always the case, fine rutile silk can enhance a sapphire’s appearance & value by illuminating darker facets of the stone. Often seen in Kashmir sapphires, these inclusions have the effect of scattering light which gives the stones their velvety appearance. 

In star sapphires, inclusions are essential to the stone’s appeal. Thus, the impact of inclusions on value depends on their type, visibility, and effect on the overall aesthetics of the gemstone. 

Collectors and gem enthusiasts often appreciate inclusions for their uniqueness and storytelling potential. Each inclusion is a record of the conditions under which the sapphire formed. These “birthmarks” are also proof of the natural origin of the gemstone which in a way signifies value. 

Inclusions can also add depth and character, creating visual interest that goes beyond simple colour and clarity. For those who value the natural origins of gemstones, inclusions are not flaws but features that enhance authenticity.