Adularescence The shimmering light or whitish opalescence which glides over the surface of some gems such as moonstone. Interference phenomena of the layered structure is the cause of this effect.
Amorphous Gemstones without a crystal structure are referred to as amorphous. These include gems such as amber, coral, opal and pearl.
Beryllium Treatment A form of heat treatment for sapphire that adds the element beryllium to the heating process. Beryllium is an element well known in the gem world, since it is an essential constituent in many gemstones, including emerald, beryl, and aquamarine. When sapphires are heated with beryllium, the result is a reduction in blue tones. Thus bright yellow or orange sapphire can be produced from weak yellow or greenish gems. Some stunning colors have been produced using this method.
Birefringence Some gemstones are singly refractive: they have only one refractive index. Other gemstones (in fact, most) are doubly refractive: they have two different refractive indices. When a beam of light enters a doubly refractive gem, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Birefringence is a measurement of the difference between the two refractive indices in gems that are doubly refractive, and it ranges from a low of .003 to a high of .287. Very few gemstones are singly refractive; in fact, the only well-known gems with that property are diamond, spinel and garnet.
Birthstone The association of gemstones with astrology goes back centuries. More recently, jewelers have adapted this tradition to create the current list of official birthstones.
Calibrated (size) Many gemstones are sold in calibrated or standard sizes that will fit commercial jewelry settings. Standard sizes are calibrated in millimeters for a number of different gem shapes.
Cat's Eye See Chatoyancy.
Center Stone The center stone is the prominent center piece in a jewelry setting that has multiple gemstones. See also Side Stone.
Chanthaburi The city in southeastern Thailand famous as one of the world centers for gemstone processing and trading. Chanthaburi is also famous for its weekend gemstone market.
Chatoyancy The cat's eye effect sometimes seen in gemstones such as chrysoberyl, apatite and tourmaline is known by the technical name of chatoyancy. The effect is caused by tiny parallel inclusions that give the appearance of a narrow line similar to a cat's eye. Often a gemstone needs to be viewed in natural light to see the chatoyancy effect.
Clarity Referring to a stone's lack of inclusions or other visual defects.
Cleavage The plane of weakness of some gems where they will split apart with smooth surfaces. Gems with perfect cleavage are likely to break when being cut or faceted.
Color Used in the evaluation of a gem. The quality of a gem can based on either the presence or the absence of color.
Color Change (gemstones) Color change gems change color due to changing light conditions (such as alexandrite or color change sapphire) or when viewed from different angles (such as andalusite or iolite).
Concave Cut Traditional gem facets are flat or two-dimensional. Concave cutting creates facets that are curved or three-dimensional. These curved facets refract more of the ambient light and return it to the eye as brilliance. Concave cutting is a recent innovation dating back to the early 1990s. It requires considerable expertise and results in higher weight loss to the rough stone, since more material must be cut away to create the curved facets.
Copper-bearing Gemstones that contain traces of copper are very rare and typically have a intense blue, blue-green or violet color. There was considerable excitement in the gem world when the first copper-bearing gemstones were discovered in 1989. See also Paraiba.
Corundum A crystalline form of aluminum oxide known in the gemstone world as ruby and sapphire. It is naturally clear, but can have different colors when impurities are present. Corundum is much admired for its hardness (9.0 on the Mohs scale), brilliance and excellent wearability.
Cubic zirconia A lab created diamond simulant, often abbreviated as CZ. While CZ is a transparent stone, trace elements can be added to the manufacturing process, producing a wide range of colors. On the Mohs scale of hardness, CZ is harder than other gemstones except for diamond, ruby, sapphire and chrysoberyl. Not to be confused with zircon, a natural gemstone.
Cuprian See Copper-bearing.
Demantoid Demantoid is a rare and valuable andradite garnet. It exhibits a range of greens from dull to bright emerald green and on rare occasions displays yellow. On the Mohs scale of hardness, demantoid is relatively soft at 6.5. It has an adamantine luster.
Density The ratio of a gemstone when compared to the weight of an equal volume of water. This means how heavy a gemstone is compared to the same volume of water. Also known as "specific gravity" for solids.
Diffusion Treatment A form of heat treatment that adds one or more chemicals to the heating process to change the color of a gemstone. Typically the treatment does not penetrate deep into the stone, so gems treated in this way cannot be recut. Diffusion treatment is a standard treatment to increase the asterism in star sapphire.
Dispersion The separation of light into the seven spectral colors, causing the "fire" of a gemstone, which is refracted by the internal facets. Diamond has a very high dispersion, hence its high amount of "fire".
Doublet A doublet is a gemstone composed of valuable gemstone material in combination with other materials. It is found most often in opal, where an opal doublet contains a slice of opal glued to common opal, glass or other material. A triplet contains a slice of opal glued between a base and a crystal or a glass top. Triplets are usually less expensive than doublets, and both are less expensive than natural opals. Doublets may occasionally be found with sapphire or other expensive gemstones.
Facet The cut and polished flat plane of a gemstone. There can be dozens of facets on a stone.
Fancy Cut Sometimes used to refer to a gemstone cut in any shape other than the standard round cut, but also used to refer to gemstones that are cut in a shape other than the well known shapes of round, oval, pear, trillion, marquise, etc.
Fire The rainbow of colors that light rays form as they move through a gemstone. This is another word for dispersion.
Fissure A surface crack on a gemstone. Gems with fissures may be fracture filled.
Fracture Filling Small cracks or fissures in a gemstone can interrupt the flow of light through the stone, creating white or "dead" spots in the color of the stone. Sometimes these fractures will be filled with material that will allow the light to pass through smoothly. Different materials are used; oil, wax, glass, epoxy, and borax are common materials. The most commonly filled stones are emerald, turquoise and ruby.
Heat Treatment The application of high heat to a gemstone in order to improve its color and clarity.
Hue Refers to the position of a color on the color wheel, or the dominant wavelength of color attributed to a gemstone. There are six primary hues: violet, blue, green, yellow, orange, and red. In between these primary hues are secondary hues, such as blue-green. See also tone and saturation.
Inclusions Foreign matter that is "included" within a stone. This may be a foreign body such as a crystal, a gas bubble or a pocket of liquid. There are many varieties of inclusions and they are important visual clues for identifying the type of gemstone and for identifying the origin of the stone.
Karat Karat (as distinguished from Carat) is a measure of the purity of gold. Most gold jewelry is actually made from a gold alloy containing gold and another metal or metals. 18K gold, for example, is 75% pure gold.
Lab created Refers to gemstones created in a laboratory rather than by nature. A lab created gemstone is typically the same material chemically as its natural counterpart, as in the case of corundum produced by flame fusion or quartz grown using the hydrothermal method.
Loupe Clean A gemstone is said to be loupe clean when no inclusions or defects are visible when the gem is viewed with 10 times magnification. See also Eye Clean.
Luster The outward appearance of a gem or organic material. The quantity and quality of light that is reflected from the surface of a stone. Luster is important especially when evaluating the quality of pearls.
Metallic (luster) One of the technical terms used to refer to the luster of a gemstone. A gemstone that is reflective like polished metal is said to have a metallic luster. Hematite is one of the rare examples.
The Mohs Hardness Scale A numerical scale ranging from 1 to 10 developed by Friedrich Mohs that assigns a rating to a gem according to its ability to resist scratching. The hardest is 10 (diamond) and the softest is 1 (talc).
Synthetic Moissanite A lab-created diamond simulant based on the structure of natural moissanite. On the Mohs scale of hardness, moissanite is 9.5. It has more brilliance, fire and luster than any hard jewel on earth, including diamond.
Oiling Oiling infuses colorless oils, resins or waxes into tiny surface-breaking fissures to hide them and give certain gemstones a cleaner appearance. This long-practiced clarity enhancement is used mainly for emerald and jade. The oils used are either natural or have a natural counterpart. If coloring agents are added to the oil, the stones are classified as dyed rather than oiled.
Opaque A term used for gemstones that light cannot be seen passing through. Lapis lazuli and malachite examples of opaque gemstones.
Organic (gemstone) Most gemstones are minerals with a crystal structure but some gems, such as amber and pearl, are organic rather than mineral, being formed by plants and animals. See also Amorphous gemstones.
Padparadscha Derived from the Sinhalese term for "lotus flower," padparadscha refers to a lush pink and orange sapphire resembling the color of the lotus. Padparadscha is also sometimes used to refer to other types of gemstones, such as topaz and tourmaline, with this unique coloration.
Paraiba A rare copper-bearing tourmaline with an intense blue or blue-green color, first found in the state of Paraiba in Brazil in 1989. There have been recent finds in Nigeria and Mozambique of similar material, and the term "paraiba" is now used to refer to all examples of this copper-bearing tourmaline. See also Copper-bearing.
Phenomenal Gems Gems that display unusual optical properties such as color change, chatoyancy, asterism or iridescence.
Pigeon's Blood Refers to the most prized color of red in rubies. Pigeon's blood red is thought to be a pure red with a hint of blue. It is associated most with rubies from Burma, though any ruby could be this color.
Portuguese Cut The Portuguese cut refers to a particular type of faceting where the gem is cut with three rows (simple cut = two rows) of rhomboidal and two rows of triangular facets above the girdle (crown) and four rows of rhomboidal and one row of triangular facets below the girdle (pavilion). The Portuguese cut thus has an extra row of facets on the crown, and this style enhances the brilliance of the gem. The Portuguese cut is one of the most popular fancy cuts in the market and you'll find many varieties of gems cut in this style.
Precious (gemstone) Traditionally, the four precious gemstones are diamond, ruby, sapphire and emerald. But other gems have also been labelled precious at times, including opal and amethyst. Today, the distinction between precious and semi-precious gems has been rejected by some gem trade associations. See also Semi-Precious gemstones.
Rubellite Used to refer to the red variety of tourmaline, including the color range from pink to red. More of a marketing than a gemological term; these days gemologists tend to use simply "red tourmaline."
Rutile Needle-like inclusions (or foreign matter) within stones. These can produce some gem phenomena such as asterism (star) or cat's eye (chatoyancy).
Saturation Saturation is one of three characteristics used to describe the appearance of color. Saturation (also known as intensity) refers to the brightness or vividness of a color. See also hue and tone.
Semi-precious (gemstone) Traditionally, the four precious gemstones are diamond, ruby, sapphire and emerald. Semi-precious gemstones include everything else. But other gems have also been labelled precious at times, including opal, amethyst and pearl. Today, the distinction between precious and semi-precious gems has been rejected by some gem trade associations. See also Precious gemstones.
Sheen This effect resembles luster, and is caused by light reflection from inclusions or texture inside the gem. Luster is light reflected from the surface of the gem and sheen is reflection from inside the gemstone.
Single Refraction Most gemstones are doubly refractive; they have two refractive indices. Only a few gemstones have a single refractive index, specifically diamond, spinel and garnet. See also Birefringence.
Species The term used to designate a family of gemstones. For example, corundum is a species that contains the varieties sapphire and ruby. The quartz family contains amethyst, citrine, and chalcedony, to name a few.
Specific Gravity see density
Synthetic (gemstone) A synthetic gemstone is man-made rather than mined from the earth. Natural gemstones that are treated by industry-accepted methods such as heat or irradiation are not classified as synthetic.
Translucent A quality of a gemstone transmitting light imperfectly so that one cannot see through the stone clearly. Star sapphire is an example of this quality.
Transparent There are several ways a light travels through a stone. In a transparent stone, the light travels through stone with virtually no distortion. Transparent stones are clear and easy to see through. See also Translucent and Opaque.
Treated stone A stone that has been heated, dyed, irradiated, or stained in order to improve the color or the clarity. Also pertains to gems that have their cracks or fractures concealed by filling the material.
Trillion Cut A faceted cut in a triangular shape with 44 facets.
Window In a well cut faceted gem, the pavilion facets (those on the lower half of the stone) should reflect light back out the top or table of the stone. If the facets are cut below the critical angle for the particular material, light will pass right through the stone instead of being reflected back toward the eye. When this happens the gem will lack sparkle and brilliance.
Zoning (color zoning) A term that describes the uneven distribution of color in a gemstone. Zoning is best seen when looking at the stone through the top table facet.
- First Published: October-23-2006
- Last Updated: September-25-2014
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