We created this section to help you understand some of the terms commonly used to describe various characteristics of precious gemstones. We will be expanding this section, so if there are specific terms you wish to learn more about, please let us know, and we will make sure to add the relevant information with our future updates.
The terms "brilliance," "fire," and "internal luster" often are used to describe the fineness of a gemstone. These rather technical and precise terms have professional meaning; they are used to measure the amount of light reflected from the interior of a gem when observed directly through the crown. Brilliance is a function of a gem's cut as well as the refractive properties of the mineral itself (measured by refractive index). The term luster is used to describe the appearance given to a gem by its light reflection. Metallic luster looks like the shining of polished metals; adamantine luster resembles glittering diamonds; vitreous minerals look more like the reflection from glass; silky luster describes the shine of malachite; resinous is for the warmth of amber; greasy is used to describe nephrite.
The group of cutting styles used for diamonds is called the brilliant cut. With this type of cut, usually it is presupposed that the lower facets radiate from the center of the gem towards the girdle. The brilliant cut has 57 facets. Its most classical shape is round, although other shapes are possible as well. More information is available in Brilliant Cut.
The word “carat” is of Greek origin and means “a fruit of the carob.” Carob seeds were used to measure different items of small size, like pearls or jewels. Now, carat is the conventional unit of mass used by jewelers worldwide. It is used to measure the volume, mass, and purity of a substance.
The metric carat (CT) equals 200mg and is used to measure the mass of pearls and gems. This standard was adopted in 1907 at the Fourth General Conference on Weights and Measures and followed soon after in many countries around the world. Nevertheless, some countries have their own carat to measure gems: the English carat equals 205mg; Arabic carat (also called qirat) is 223mg; Asia Minor qirat is 200.4mg; Egyptian qirat equals 195mg. The Egyptian carat, equal to 0.064L, is used for measuring a gem's volume.
Carat is also used for defining solidity of precious metals. For more information, please check Metals Glossary.
The most common way to get a certified gemstone is to require a GIA (Gemological Institute of America) certificate when buying jewelry. Usually, a certificate is a document defining a gemstone's type, weight, color, clarity, proportions, finish grade, treatments, etc. In the United States, GIA reports are considered the most trustworthy certification. Some jewelry shops have their own accredited labs, which can certify your purchase according to the parameters mentioned above. They can also identify some other features, such as a gem's hardness or its appraisal value.
Four main characteristics are used to evaluate a precious stone: weight, cut, color, and clarity. The clearer the stone is, the more carefully it is cut, the more it weighs, and the more unique color it possesses, the more the stone is valued.
The term "clarity" in relation to a gemstone usually is defined by two characteristics: the absence or presence of any inclusions and the degree to which light can pass through a stone. These two characteristics of clarity are interlinked; the presence of inclusions can affect the way light passes through the gem. Typically, all gemstones are graded according to the presence and visibility of the inclusions.
Some gemstones, like diamonds and other beryls, are valued for their transparency and clarity, and the presence and visibility of the inclusions can significantly reduce their price. On the other hand, there are gems like emeralds or rubies, which are valued for their color, and the more inclusions these gems have, the deeper is their color.
The visibility of inclusions is usually measured in gemstones which are valued for their clarity. The more visible the inclusions are, the smaller is the price of the gemstone. The grading of inclusions visibility is done on a separate scale. You can read more about this topic in the article about diamond's clarity.
A feather usually is an internal flaw of a specific nature, such as an inclusion or a crack going deep into the stone's interior. Feathers and cleavages can start at the surface of the stone and extend deep inside, or sometimes they may occur within the stone as the result of changes in atmospheric pressure. Feathers may either ruin a stone by making it fragile or by changing its color or clarity, or they may increase its value and beauty by adding to its uniqueness.
Color is the most obvious and attractive feature of gemstones. The color of a specific stone is due to the atomic structure of the stone and its impurities, which absorb certain colors and leave other colors unaffected.
For diamonds, the term "color" refers to some position in the color scale devised by the Gem Trade Laboratory of the Gemological Institute of America (GIA). The colors are divided alphabetically from D to Z. Letters D through M are used with clear, colorless diamonds: D is absolutely colorless; letters N through Z describe various degrees of yellow, and Z is given to the fancy yellow color. This scale is recognized worldwide. To learn more about diamond color, please check the article about diamond color.
The term gemstone cut usually refers to the proportions and general shape of a stone, the number and positioning of the faces (usually called facets), and the type of final polish. Rough gemstones are cut in order to allow the best light play, which is one of the most important characteristics of a precious gem. There are numerous cutting styles for various gems, used to enhance the natural qualities of each gem. Cutting styles can be divided into two main groups - the diamond (or brilliant) cut and the step-cut. More information about cut types is available in the article about diamond cut.
Crystal inclusions, sometimes called feathers, are one variety of inclusions that may occur in gemstones. They often add to a stone's value. Topazes may include golden threads; rubies may include various crystals that look like stars. The most popular inclusion-holders are the quartzes. Gemologists classify inclusions into three groups according to when they were formed. One group contains those inclusions which were formed before the hosting gemstone was formed. Inclusions belonging to this category usually are of a crystal nature; often, they are desirable and considered valuable, enhancing the price and look of the gem. Another group is for inclusions that formed at the same time as the host crystal, so they were imprisoned within the gem, and the last group is comprised of inclusions that formed after the gem was grown.
Dispersion is an optical property that measures the way colored light is partially returned to the surface when reflected from within the stone's interior. Sometimes, jewelers refer to this property as "fire." White light passes through a gemstone and splits into rays of various colors. Because of this feature, some stones can play light in very elaborate and intricate ways. Light dispersion can be measured with a refractometer; refractive index is the measure of the speed and angle of light entering a gemstone. Dispersion is the difference between the red and violet indices.
The cut and polished, flat plane of a gemstone is called the facet. The number of facets, their arrangement, and the angles between them may differ among various types of cuts and types of stones.
Faceting is sometimes a synonym for the term "cutting." It is the process of making facets on a stone's surface. Faceting is common in transparent stones; it enables light to pass through a stone at exact angles.
Fancy color is a term used to describe colored diamonds. The coloring can be caused by various factors. Some chemical elements presented to stones can add a tint. For example, nitrogen creates a yellow diamond. Color can also be caused by radiation that was present during the diamond's creation. The green diamond hues are usually the result of radiation. Sometimes, color is the result of inclusions, which can add tones and flashes of color in a fancy color diamond.
Any kind of shape but the classical brilliant cut can be regarded as fancy. Marquise, Pear, Princess, and Heart shapes are the most popular variations of fancy cuts.
Fluorescence usually is defined as the emission of visible light from a mineral or gemstone when it is stimulated by radiation or ultraviolet light. It is a common feature mentioned when referring to gemstones, especially white ones. Sometimes, fluorescent properties help to identify the presence of hue in a diamond, which can reduce its value. The most valued diamonds are the colorless, white ones.
The Gemological Institute of America was established in the 1931. It is the world's most respected laboratory, entrusted with grading and identifying more gems than any other laboratory in the United States. The institute also serves as the most trusted knowledge base of diamonds, colored stones, and pearls.
Hardness is the characteristic that measures a mineral's sustainability and durability against scratching. It is measured on Mohs hardness scale of 1 to 10. Accordingly, a diamond, being the hardest, is 10 points; ruby and sapphire are 9 points; and malachite is only 3 points.
The gemstones' inclusions can range from atomic (also known as impurities) to organic. It may be an admixture that turns a ruby to sapphire, it may be a solid quartz crystal trapped in the host stone, or it may even be an ancient insect or a piece of plant trapped in amber. Some inclusions are viewed as bad and unattractive, reducing the gem's value, while others make the mineral look unique, beautiful, and thus more expensive.
Mohs scale of mineral hardness was created by the German mineralogist Friedrich Mohs in 1822 to measure the relative hardness or scratch resistance of various minerals. The scale grades hardness from 1 to 10: a diamond, being the hardest, is 10 points; ruby and sapphire are 9 points; and malachite is only 3 points.
Being too small to measure in carats, the weight of smaller gemstones is often measured in points. One carat is equal to 100 points, e.g. a 10-point diamond has the weight of 0.1 carat.
The term pointer usually refers to any gemstone which weighs less than 0.01 carat. They are usually polished or faceted, and their weight is expressed in points, i.e. in 1/100 of carat.
This index measures the speed and the angle of light entering a gemstone. Each gemstone has its own unique refractive index.
Gem setting refers to the specific mounting used to secure the gem in a jewelry piece. Depending on the size and shape of a gemstone, the desired look of a jewelry piece, and the jeweler's artistic vision, different gemstones require different settings. The major objectives of any type of setting are to secure the gemstone and to enhance its most valuable features, such as the color, light play, and shape. The most common gem settings are the bezel, pave, bright cut bid setting, and prong. More detailed information is available in Gem Settings.