The distinct beauty, color, and sparkle of various gemstones stem from their unique physical and optical characteristics. These properties are meticulously analyzed by gemologists to determine a gemstone's origin and formation process.
While some dedicate their entire careers to this field, grasping a fundamental understanding of gems isn't challenging and can greatly enhance the experience of appreciating gemstones. The captivating beauty of a gemstone, its interaction with light, and its hue all originate from these inherent properties, which we will look at comprehensively in this article.
Color
Optics, the study of light and its transmission through waves, determines how we perceive color. Light waves vary in length, measured in Angstrom Units, with visible light ranging from red (7600 Angstroms) to violet (4000 Angstroms). Sunlight, containing all light components, appears as white light, but when dispersed by a prism, it reveals a color spectrum.
This spectrum influences the color of gemstones, which absorb and reflect specific wavelengths, affecting their appearance. When examining gemstones, the type of light used is crucial. Professional buyers prefer indirect daylight for its natural quality, but in jewelry stores, gemstones are often displayed under intense artificial light to enhance their beauty.
However, this can alter their appearance compared to natural light, with some stones like alexandrite changing color entirely under different lighting conditions. Sapphires and tanzanite are also affected by lighting, showing different hues under various light sources. Therefore, it's important to view gemstones under multiple lighting types, especially the light in which they will be worn, to appreciate their true color and brilliance.
Refractive Index
Refraction occurs when light bends entering a new medium, like a stick appearing broken in water. Each gem has a characteristic refraction rate, known as the refractive index, measured using a refractometer, a vital tool for gemologists. The higher the refractive index, the more brilliant a gemstone can be, especially if transparent and well-cut. Gems forming in the isometric system are singly refractive, bending light uniformly. In contrast, gems from other crystal systems are doubly refractive, splitting light into two rays at different rates. Diamonds and zircons, with high refractive indices, are known for their exceptional brilliance.
Birefringence
Birefringence in gemstones refers to the difference between two refractive indices found in doubly refractive gems. While most such gems have similar refraction rates, some exhibit significant variations, making them highly birefringent. Notable examples include zircon and peridot. Birefringence is observable under magnification, where elements like the back facet junctions of a zircon or peridot appear doubled. This optical feature not only distinguishes these gemstones but also imparts a unique velvety appearance, subtly softening their color and enhancing their visual appeal. This characteristic is a fascinating aspect of gemology, contributing to the attractiveness and uniqueness of these stones.
Pleochroism
Pleochroism is the phenomenon where doubly refractive gemstones absorb light at different rates along their crystal axes, causing them to display varying colors from different angles. For example, a dark green tourmaline may appear green from one end and brown from the side, while iolite can look blue from one viewpoint and colorless from another. Tools like the dichroscope help observe this effect. Gems with two pleochroic colors are dichroic (e.g., tourmaline, amethyst, rubies, sapphires), while those with three are trichroic (e.g., tanzanite, andalusite). This attribute adds depth and richness to a gemstone, making it essential to view pleochroic gems from multiple angles to appreciate their full color range.
Dispersion
Dispersion occurs when a gemstone fractures light into a full spectrum, creating rainbow-like flashes. Gems like diamonds and zircons display high dispersion, enhancing their brilliance. Rare stones such as benitoite, demantoid garnet, and sphene are known for their exceptional dispersive qualities. The cut of a diamond critically influences its dispersion; a well-cut diamond showcases a fiery appearance, while a shallow cut minimizes dispersion, diminishing its full aesthetic potential.
Fluorescence
Some gemstones exhibit fluorescence under ultraviolet light, absorbing UV radiation and emitting visible light. This effect is noticeable in sunlight, which contains UV light, making highly fluorescent gems briefly flash. Around 30% of diamonds fluoresce, typically blue, though yellow, green, and rare pink fluorescence also occur. Strong fluorescence can give diamonds an "oily" appearance in sunlight, reducing their appeal. However, moderate blue fluorescence can enhance a diamond's beauty, making slightly yellow diamonds appear more colorless and thus more valuable. Rubies also fluoresce, with Burmese rubies known for their higher fluorescence, contributing to their desirability and value.
Transparency
Transparency in gemstones ranges from transparent, like clear glass, to opaque, blocking all light. Semi-transparent stones allow partial visibility, while translucent ones let light pass but obstruct clear vision. This characteristic is crucial in evaluating gemstones; for instance, chrysoprase is typically translucent and lapis lazuli opaque. Transparency directly affects a gemstone's value, particularly in jadeite, where higher transparency significantly increases its worth. The clarity and amount of light a gem allows through greatly influence its appeal and market value.
Toughness and Durability
A gemstone's durability depends on factors like hardness, internal structure, and cleavage. For instance, nephrite jade, though moderately hard (6 1/2 on Mohs scale), exhibits excellent toughness due to its fibrous structure. Diamonds and topaz, despite high hardness, possess perfect cleavage, making them susceptible to splitting under certain impacts. Internal fractures and brittleness, common in garnets and opals, necessitate careful setting. Gems like peridot and zircon, prone to surface abrasion, require protective settings. When selecting a gemstone for jewelry, especially rings, considering its overall toughness is crucial to ensure appropriate setting, wear, and maintenance.
Hardness
Hardness in gemstones, indicating their resistance to scratching, is measured on the Mohs scale, which ranges from one to ten. This scale isn't evenly graduated; the gap in hardness between corundum (9) and diamond (10), the hardest known natural material, is significantly larger than between other minerals. Additionally, some minerals exhibit varying hardness within a single crystal based on the direction of the scratch.
The role of hardness in determining a gemstone's overall toughness is crucial, but it's not the sole factor. Diamonds, at the pinnacle of the Mohs scale, are renowned for their exceptional hardness, making them nearly scratch-proof. However, this extreme hardness doesn't equate to indestructibility. Rubies and sapphires, with a hardness rating of 9, are known for their durability and are excellent for regular wear.
In contrast, topaz, rated at 8 on the Mohs scale, resists scratching but is prone to cleavage, meaning it can split easily and requires careful handling and setting. Generally, gemstones with a hardness of 7 or above are less prone to scratches, making them suitable for rings. Stones softer than 7 can be less durable for such purposes, although there are exceptions like nephrite jade, which combines moderate hardness with excellent toughness.
Luster
Luster refers to how a gemstone's surface appears due to the way it reflects light. It can be categorized as either metallic or non-metallic, with non-metallic lusters described as follows:
l Dull: Does not reflect light
l Resinous: Similar to the appearance of resin
l Greasy: Looks like it has an oily surface
l Silky: Gives off a fibrous reflection
l Pearly: Shows an iridescent, pearl-like effect
l Vitreous: Has a glassy appearance
l Adamantine: Possesses a brilliance akin to a diamond
The luster is a key element in a gemstone's allure, and achieving optimal luster involves careful polishing and finishing. This process enhances the natural beauty and the reflective qualities of the gem.
Specific Gravity
Specific gravity measures a substance's density compared to water at 4°C. It's expressed as a decimal, e.g., 4.00 for corundum, 3.52 for diamond, and 2.72 for quartz, varying slightly with trace elements. This is crucial in gem buying, as gems are sold by weight, but mountings are designed for standard sizes. For example, a 6.5 mm diameter diamond typically weighs 1 carat, while an amethyst of the same size weighs about 0.80 carats, and a sapphire or ruby, 1.25 carats. Thus, replacing a gemstone with another of the same size will result in a different weight due to varying specific gravities.
Inclusions
Inclusions in gemstones, such as internal crystals, fractures, and bubbles, can vary from being microscopic to visible to the naked eye. Flawless gems or those with minimal inclusions are typically more valuable. While internal fractures can weaken a gemstone, not all inclusions are detrimental; they can aid in identifying and tracing the origin of a gem. In diamonds, inclusions significantly affect clarity grading and price. However, in colored gemstones, the impact of microscopic flaws on price is lesser. The key is assessing whether inclusions harm the stone's beauty or visibility.
Inclusions can affect a gemstone's transparency, a critical quality factor. For example, a Kashmir sapphire with minute inclusions may not lose value if it has exceptional color, and these inclusions can even verify its origin, enhancing its worth. Additionally, some inclusions contribute to unique optical phenomena, increasing a gem's value.
Phenomenal effects, like asterism (star effect) and chatoyancy (cat's-eye effect), are caused by the way light interacts with inclusions. Asterism is best observed under concentrated light, revealing star patterns common in rubies, sapphires, and occasionally in spinels. Chatoyancy creates a cat's-eye appearance, seen in stones like chrysoberyl and moonstone. In moonstone, adularia produces a captivating flash, adding to the gemstone's allure.