When selecting a gemstone, it is vital to understand its terms and structural parts. Whether purchasing an engagement ring, wedding band, or picking out a beautiful gemstone, understanding these concepts will help you make better choices in color, cut, and optic effects. This article will give you a detailed overview of the parts of a gemstone and their associated terms, helping you to gain a deeper understanding of these fascinating natural or lab gemstones.
Table of Contents:
Parts of a Gemstone
Different gemstone types and cutting styles significantly influence a gemstone’s appearance, optical properties, and visual impact. Although various gemstones have distinct appearances, their basic components are similar. Let’s explore the structure of a gemstone using the most common “round brilliant cut” as an example.
Table
The table is the largest flat surface on the gemstone, located at the top, and it’s where light first enters the gemstone. After light enters through the table, it reflects and refracts across other facets, ultimately exiting through the table or other facets. The size of the table directly affects the gemstone’s brilliance and fire (fire refers to the rainbow-like flashes reflected from the gemstone). A larger table increases brightness but may reduce fire. In step cuts like the emerald cut, which features a larger table, clarity becomes crucial as inclusions are more visible.
Facets
Facets are the flat surfaces on the gemstone, created through cutting and polishing, including the table. Their main purpose is to reflect and refract light as it passes through the gemstone, enhancing its sparkle. The arrangement, design, and number of facets determine a gemstone’s optical effects; the more precise the cut, the better the light performance. (However, more facets do not always mean better quality. Poor facetting can cause light leakage, making the gem appear dull.)
Facet Types:
- Crown facets: Located on the upper part of the gemstone, including the table, triangular facets, star facets, kite facets, etc.. These facets direct light into the gemstone.
- Pavilion facets: Found on the lower part of the gemstone, including pavilion main facets and culet facets. Their main function is to reflect light out, making the gemstone sparkle.
- Girdle facets: Located at the girdle of the gemstone, connecting the crown and pavilion. These facets enhance the gemstone’s strength and aesthetics, also assisting in light distribution within the gemstone.
- Culet facet: This is a small facet at the very bottom of the gemstone, concentrating the light reflected from the pavilion to enhance brightness. Not all gemstone cuts have a culet facet, as some cuts, like the emerald cut, which emphasizes clarity and color, do not include one.
Crown
The crown is the area of the upper part of the gemstone that encompasses the area from the girdle to the top of the table. The height of the crown determines the angle of reflection of light and the expression of fire. A deep-cut crown will increase the refractive path of the light and enhance the fire, while a shallow-cut crown will make the gemstone appear flatter and detract from the effect of the light. The design of the crown is the main focus of the gemstone cutter and has a direct impact on the beauty and visual impact of the gemstone.
Deep cut: Depth > girdle, appears taller.
Shallow cut: Depth < girdle, appears flatter.
Girdle
The girdle is the widest part of the gemstone, located between the crown and the pavilion. It affects the size and weight of the gemstone. The thickness of the girdle is critical to the durability and optical properties of the gemstone. If the girdle is too thin, the gemstone will be easily damaged; while a girdle that is too thick will affect the brightness and overall proportions of the gemstone. Ideally, the girdle should be neither too thick nor too thin, just enough to protect the gemstone without compromising its optical properties.
Pavilion
The pavilion is the lower part of the gem, extending from the girdle to the culet. Its main function is to guide light through the gem, ensuring that it is reflected from the table or other facets. The depth of the pavilion affects the reflection of light; a pavilion that is “too deep” will allow light to pass through the gem and escape, resulting in a dull appearance, while a pavilion that is “too shallow” will make the gem appear less sparkling.
Culet
A culet is the lowest facet of a gemstone that gathers light and enhances its brilliance. Not all gemstones have a culet. Some cuts, such as the brilliant cut, have a very small culet, while the step cut, which emphasizes clarity, usually does not have a culet. Ideally, the point should be very small to avoid light leakage, but if it is too large, it may appear visually imperfect.
Common Gemstone Terms
Gemstone terminology covers the optical properties of gemstones, the quality of cut, fire and brilliance. Here are some common gemstone terms that can help you better understand the qualities of a gemstone.
Gem 4Cs
The 4Cs of gemstones are the color, clarity, cut, and carat weight of a gemstone, and are the 4 key indicators of a gemstone’s quality.
Depth
Depth is the vertical height of the gemstone from table to culet and significantly affects the brilliance and fire of the gemstone. Too much or too little depth will affect the reflection of light from the gemstone and cause it to lose its luster. The ideal gemstone cutter will have a moderate depth that maintains sufficient light reflection without excessive light leakage.
Symmetry
Symmetry refers to the uniformity of a gemstone’s facets. A perfectly symmetrical gemstone reflects light more evenly and enhances the overall aesthetics of the gemstone. A poorly cut gemstone with unevenly aligned facets will reflect light unevenly, affecting its brilliance and sparkle. Ideally, a gemstone should be rated at least Good or Very Good for symmetry.
Polish
Polish is an assessment of the smoothness of a gemstone’s surface. A good polish makes the surface of the gemstone smoother and enhances its brilliance and fire. A poorly polished gemstone may have scratches, pits, or polish lines that affect the reflection of light. Polishing grades are usually assessed together with symmetry and are categorized as Excellent, Very Good, Good, Fair Good, etc.
Fire
Fire refers to the colored patches of light that a gemstone produces by refracting light. It is a highly valued characteristic, especially in high quality cut gemstones. A gemstone’s fire is closely related to the number and arrangement of its facets. Larger cuts (such as emerald cuts) tend to have a lower fire, while round brilliant cuts tend to have a higher fire due to the number of facets.
Brilliance
Brilliance is the intensity of white light reflected by a gemstone. The brilliance and sparkle of a gemstone is one of the keys to its appeal. A gemstone’s brilliance is seen as the white light that flashes when the gemstone is moved. Gemstones with strong scintillation are usually finely cut to maximize the use of light. Gemstones with a brilliant cut usually have a very high degree of scintillation, while those with a step cut have a relatively low degree of scintillation.
Bow-Tie Effect
The bow-tie effect is a phenomenon that occurs in certain fancy cut gemstones, particularly in oval, pear, heart and marquise cuts. Due to the different arrangement of facets in these cuts, a bow-tie-like shadow can appear at certain angles. This effect affects the reflection of light in the gem, and a good cut will minimize the bow-tie effect and keep the brightness and scintillation of the gem consistent.
Light Leakage
Light leakage is an area within a gemstone that only absorbs light and does not reflect it. This is usually caused by poor cutting. Too much light leakage can make a gem appear dull. Therefore, it is important to choose a gemstone with a good cut that reduces light leakage and improves the optical effect of the gemstone.
Refractive Index (RI)
The refractive index refers to the degree to which light is bent as it passes through a gemstone. Different gemstones have different refractive indices. The higher the refractive index, the more intense the reflection of light from within the gem, and the greater the brilliance and fire. For example, diamonds have a high refractive index (about 2.42), which allows them to display intense sparkle and fire. In contrast, a gemstone with a lower refractive index, such as quartz (about 1.54), has less fire and scintillation.
Dispersion
Dispersion refers to a gemstone’s ability to disperse white light into a spectrum of rainbow colors. The greater the dispersion, the richer the fire of the gemstone. Diamonds are known for their high dispersion (approximately 0.044) and their ability to reflect a rainbow of brilliant light. Dispersion is also an important indicator of a gemstone’s visual appeal, and high dispersion gemstones tend to perform particularly well in bright light.
Dichroism
Dichroism refers to the fact that a gemstone shows two different colors when viewed from different angles. This phenomenon is common in certain gemstones, such as tanzanite and sapphire. Gemstones that are dichroic can change color when viewed in different light or from different angles. For this reason, cutters often have to pay special attention to the direction in which these gemstones are cut to reveal their best colors.
Pleochroism
Pleochroism refers to the fact that a gemstone can show two or more different colors when viewed from different angles. It is similar to dichroism, but pleochroism allows for the display of more colors. Gemstones such as andradite garnet and alexandrite are highly pleochroic, making their colors unpredictable and attractive.
Luster
Luster refers to the degree and manner in which light is reflected from the surface of a gemstone. Different types of luster give gemstones different visual effects. Common types of luster include:
- Glass luster: the surface of the gemstone is smooth and shiny like glass (e.g. quartz).
- Silk luster: a soft luster that looks like the surface of silk (e.g. star sapphire in sapphires).
- Metallic luster: a strong metallic-like reflection (e.g. pyrite).
Inclusions
Inclusions are natural imperfections or features that form inside or on the surface of a gemstone, such as bubbles, droplets or mineral crystals. The presence of inclusions affects the clarity and optical properties of a gemstone. While many inclusions can diminish the quality of a gemstone, in some cases unique inclusions can add value to a gemstone, such as in the case of the cat’s eye effect, where inclusions are one of the reasons for the light effects it produces.
Clarity
Clarity is a measure of the presence or absence of inclusions and blemishes within and on the surface of a gemstone. Usually categorized into several levels, ranging from Flawless (FL) to Included (I), which has a high number of inclusions and blemishes. Flawless gemstones are usually rarer and more expensive, while those with more inclusions are less expensive.
Fluorescence
Fluorescence is the light that a gemstone emits when exposed to ultraviolet light. Different gemstones appear to have different colors or brightness when exposed to fluorescence. For example, a diamond may fluoresce blue or yellow under UV light. Fluorescence can affect the appearance of a gemstone in natural light; for example, some highly fluorescent diamonds may appear brighter or softer.
Specific Gravity
Specific gravity is the density of a gemstone and is often used to differentiate between gemstones that are similar in appearance but different in material composition. For example, a diamond has a specific gravity of 3.52, while a similar looking white sapphire has a specific gravity of 4.00, making them feel very different. A gemstone with a higher specific gravity will feel heavier for the same volume.
Hardness
Hardness refers to a gemstone’s ability to resist scratching or abrasion. It is commonly measured on the Mohs scale, on a scale from 1 (the softest talc) to 10 (the hardest diamond). For example, diamonds, with a hardness of 10, are the hardest natural substance, while topaz, with a hardness of 8, is relatively hard but still far less durable than diamonds. Hardness is particularly important in everyday wear and determines the durability and suitability of a gemstone for its intended use.
Blemish
Flaws are imperfections or defects in the surface of a gemstone, such as scratches, pits, or fractures in the faceted edges. Blemishes differ from inclusions in that they affect only the appearance of the gemstone, not its internal structure. While flaws can detract from a gemstone’s appearance, they do not affect its optical properties as much as flaws do.
Asterism
Asterism is an optical phenomenon usually found in certain types of gemstones, such as star sapphires and star rubies. Due to the very regular arrangement of elongated inclusions or structures within the stone, asterism are formed when light strikes the stone at a particular angle. The starburst effect is highly prized, especially in high quality gemstones.
Chatoyancy
The cat’s eye effect is a unique optical phenomenon found in certain gemstones, such as cat’s eye or tiger’s eye. It is caused by fibrous inclusions or structures within the gemstone, and when light hits the gemstone, a bright band of light appears, like a cat’s eye.
Saturation
Saturation is the term used to measure the intensity and purity of a gemstone’s color. Highly saturated gemstones have bright, intense colors, while low saturation gemstones appear muted or grayish in color. The saturation of a gemstone plays an important role in determining its visual appeal and value.
Iridescence
Iridescence is a phenomenon usually found in gemstones such as opal or labradorite. It is a color change caused by the interference of light rays on the surface of the gemstone or on tiny structures within it. The banding effect makes the gem appear colorful, especially when viewed from different angles, and the colors are constantly changing.
Conclusion
By familiarizing yourself with these gemstone terms and the parts of a gemstone, you can better assess a gemstone’s quality, cut and optical properties. Whether you are purchasing an engagement ring or selecting a gemstone for your personal collection, this knowledge will help you make a more informed decision and ensure that you purchase the right gemstone for you. By understanding these basic concepts, you will be able to appreciate and purchase stunning gemstones with greater confidence and enjoy their unique appeal and value.