Sapphire, a gemstone cherished for its beauty, holds significant value. Accurate identification is essential to distinguish it from imitations or other natural stones. Understanding sapphire’s unique characteristics helps assess its authenticity and value, preventing confusion.
Visual Clues to Sapphire Identity
Sapphire exhibits a broad spectrum of colors beyond classic blue, including pink, yellow, green, purple, orange, and black. Some display color-change phenomena or unique peach-orange hues known as Padparadscha. Strong to vivid color saturation is highly desired.
Clarity in sapphires often involves various internal features, known as inclusions. Common types include fine, needle-like rutile inclusions, often called “silk,” and partially healed fractures resembling fingerprints. While a flawless sapphire is rare and might suggest a synthetic origin, some inclusions, like silk in Kashmir sapphires, can increase value by giving a velvety appearance.
Sapphires typically display a vitreous, or glass-like, luster. They are generally transparent to translucent.
Natural sapphires may also show color zoning, where color is not uniformly distributed but appears in bands or patches. This characteristic is rarely seen in lab-grown counterparts.
Testing Sapphire’s Physical Properties
Sapphire possesses notable hardness, ranking 9 on the Mohs scale, just below diamond. This makes it highly resistant to scratching and a durable choice for jewelry. However, scratch tests should be approached with caution as they can damage softer materials and may not be definitive.
Sapphire has a relatively high specific gravity, typically ranging from 3.95 to 4.03. This density means a sapphire will feel heavier than many other gemstones of similar size. While precise measurement requires specialized equipment, a noticeable heft can provide an initial clue.
Sapphire also has a specific refractive index (RI), generally falling between 1.76 and 1.77. This property describes how light bends when passing through the stone. While a refractometer is needed for accurate measurement, many sapphires also exhibit dichroism, appearing to show different colors when viewed from different angles. Some sapphires may fluoresce under ultraviolet (UV) light, though this is not a definitive test.
Separating Sapphire from Other Stones
Distinguishing natural sapphire from other gemstones involves assessing its visual and physical properties. Blue topaz is generally softer and has a lower specific gravity. Tanzanite exhibits stronger pleochroism, showing distinct color changes from blue to violet or reddish hues depending on the viewing angle, compared to sapphire’s less dramatic dichroism. Spinel, while hard, typically has a lower refractive index and specific gravity. Glass, a common imitation, is much softer, often shows gas bubbles, and lacks the internal inclusions or optical properties of natural sapphire.
Synthetic sapphires, created in laboratories, share the same chemical composition and crystal structure as natural ones, making visual differentiation challenging. Lab-grown versions often lack the natural inclusions found in mined sapphires, appearing remarkably clean. Some synthetics, particularly those produced by the flame fusion method, may exhibit curved growth lines or gas bubbles, features not present in natural stones. Natural sapphires, in contrast, typically show angular growth patterns and a wider variety of inclusions, such as silk or fingerprints.
Sapphires can undergo various treatments to enhance their appearance. Heat treatment is common, improving color and clarity by dissolving certain inclusions. While heat-treated sapphires often have more uniform color, changes to inclusion patterns, such as “heat-altered inclusions” or “discoid fractures,” can indicate treatment.
Diffusion treatment introduces color-causing elements into the stone’s surface at high temperatures, resulting in a thin layer of color. This can sometimes be identified by color concentrations along facet junctions or by immersion in a liquid with a refractive index similar to sapphire.
Knowing When to Seek Expert Advice
While home observation and simple tests offer initial clues, definitively identifying a sapphire and determining treatments can be complex. The subtle differences between natural, synthetic, and treated stones often require specialized knowledge and equipment. For valuable gemstones or when uncertainty remains, consulting a certified gemologist is a practical step.
Gemologists possess advanced tools like refractometers for precise refractive index measurements, microscopes for detailed inclusion analysis, and spectrometers for chemical composition analysis. Their expertise allows for accurate identification and assessment of a sapphire’s origin and any enhancements, providing assurance of its true nature and value.