Gemology Definitions & Terminology

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Adamantine (a) Brilliant light reflecting and transmitting properties in diamonds and other gemstones, known as adamantine lustre.

Adularescence (n) A white or bluish reflection coming from a definite plane in a mineral when the stone is turned. The name comes from the feldspar variety adularia and is best known in the gemstone moonstone.

Allochromatism (a) Coloration caused by the presence of a trace element or impurity that is foreign to the crystal lattice.

AGS (n) American Gem Society (AGS) An industry trade association of professional gemologists based in Las Vegas, Nevada. AGS was founded in 1934 by Robert M. Shipley.

Allotrope (n) An element which exist in two or more different forms in the same physical state. Diamond and Graphite are both allotropes of carbon yet diamond is hard and graphite is soft. Diamonds crystallize in the cubic crystal system, and Graphite crystallizes in the hexagonal system.

Alluvial (a) A gemstone that is created from soil or sediments deposited by a river or other running water.

Amorphous (a) Amorphous materials are the result rapidly cooling molten material. The cooling reduces the mobility of the material’s molecules before they can pack into a more thermodynamically favorable crystalline state.

Anisotropic (a) A doubly refractive gemstone such as synthetic Moissanite or natural Zircon.

Asterism, Asteriated (n, a) Optical phenomenon displayed by some rubies, sapphires that disperses reflected light in the shape of a star.

Beryl (n) Beryl is a mineral made up of beryllium aluminium cyclosilicate. Pure beryl is colorless, colored beryl has been tinted by impurities; Green beryl is called emerald, blue beryl is aquamarine. The name comes from the Greek beryllos for the blue-green color of sea water.

Biaxial Gemstones (a, n) Biaxial gemstones have two optical axes that do not line up to the c-axis. In uniaxial stones, the optic axis is parallel to the c-axis.

Birefringence (n) In a anisotropic or doubly refractive mineral, birefringence is the difference in values between the highest refractive indices and lowest refractive indices.

Brilliance (n) A quantitative measure of the amount of light return through the gem. A GIA Standardized term.

Brilliant Cut (n) Introduced in the middle of the 17th century, a round brilliant cut is used when the diamond crystal is an octahedron composed of eight equilateral triangles. Two stones can be cut from one such crystal.

Bruting (v) Cutting the rough to the round girdle outline on a brilliant cut.

Cabochon (n) Gemstone shaped and polished into an oval as opposed to facetted. Appropriate for most opaque and/or soft gems.

Calcite (n) Calcite is a calcium carbonate and one of the most widely distributed minerals on the Earth’s surface. It is a common constituent of sedimentary rocks such as limestone.

Chalcedony (n) Chalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy (semi-gloss) luster.

Chatoyancy (v) Derived from the French word, meaning ‘cat’s eye,” chatoyancy is an optical reflectance effect seen in certain gemstones. Chatoyancy is caused by the fibrous structure of a mineral or gemstone such as “tiger eye” quartz.

Chiastolite (n) A metamorphic mineral, Chiastolite is a variety of Andalusite. It contains cross-shaped inclusions of carbon.

Cleavage (n) Describes an internal crack or break in a gemstone along a “cleavage plane”. Also describes the tendency of a crystal to split along the grain. The bonding between atoms is weaker along certain planes within the crystal. Described as: Perfect (easy), Distinct, and Indistinct.

Colluvium, Colluvial (n, a) Loose deposit of rock debris that has accumulated at the base of a cliff or slope. Deposited by sliding or falling (gravity).

Craton (n) Portions of the earth’s crust that have remained stable for at least 1,500 million years.

Cryptocrystalline (a) Cryptocrystalline describes a rock’s texture which is so finely crystalline that its crystalline nature is only visible microscopically by viewing a thin section by transmitted polarized light.

Crystal (n) Crystal is a solid symmetrical structure in which the atoms and molecules are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.

Crystal Habit (n) Habit refers to the way in which a crystal’s atomic structure determines the external shape of the crystal.

Dichroic (a) A crystal that appears to change color when viewed from different angles. Similar to polarization.

Dispersion (n) Describes the way a diamond breaks up a ray of white light into its component colors.

Dop (n) Also known as a “Dop Stick” or “Dop Chuck”, a dop is a rod used to hold the rough gemstone during the grinding process. The rough is usually held to the dop with wax glue.

Druzes (Druzi) (n) A bed has a very fine crystalline structure similar to sugar or salt crystals in appearance and texture. Aggregate of minute crystals coating a surface.

Eluvium, Eluvial (n, a) Residual secondary deposits of soil and rock particles transported and re-destributed by wind.

Fancy cut (a, n) Macles and other oddly shaped crystals are cut in a fancy cut in order to minimize waste and follow the rough shape of the stone.

Facet (n) Flat surfaces and planes in cut in geometric shapes. A faceted cut is used to create a gemstone that sparkles with internally reflected light.

Feathers (n) Tiny cracks within the stone that resemble feathers. Generally, feathers do not affect durability unless they reach the stone’s surface.

Fire (v) Portions of light returning through the gem that are dispersed into the spectrum of rainbow colors or “fire”. A GIA Standardized term.

Fluorescence (v) Blue, yellow or white glow seen on a diamond when viewed in ultra-violet light or direct sunlight.

Fossilization (n) In order for an organism to be fossilized, the remains normally need to be covered by sediment as soon as possible. Additionally, an organism can become petrified when it comes to rest in an anoxic (oxygen-free) environment such as at the bottom of a lake or sea-bed.

Fracture (n) When a mineral contains no planes of weakness (cleavage), it will break in random directions.

Fracture Filling (n) Injecting colorless plastics or glass into fractures and inclusions to hide imperfections.

Gabbro (a) A dense, coarse-grained greenish dark colored rock igneous rock that is chemically similar to basalt.

Gemmy (a) A stone that glitters like a gem.

GIA (n) Gemological Institute of America (GIA) is the world’s foremost authority in gemology, diamond grading, jewelry education, gemology research.

Girdle (n) The flattened edge of a diamond or precious gemstone.

Girdle Fringes (n) Sometimes referred to as “Bearding”, Girdle Fringes are hair-like lines that can occur around the girdle during the cutting process. They can be removed by polished or re-cutting.

Gneiss (n) Part of the later stage of the “rock cycle” (sedimentary > metamorphic > igneous), gneiss (pronounced “nice”) is a type of high-grade metamorphic that is coarsely foliated and recrystallized but does not carry large quantities of micas or other platy minerals. “Orthogneiss” designates gneiss derived from an igneous rock, and “Paragneiss” is derived from a sedimentary rock.

Grain (n) Visible evidence of the crystal structure of a diamond. See “cleavage.”

Idiochromatism (v) The presents of essential or major constituents within the mineral’s crystal lattice determine which wavelengths of light are reflected and which are absorbed, determining color.

Igneous rock (a, n) Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

Inclusion (n) A visible foreign substance (flaw), fracture, or void inside a diamond or gemstone. Inclusions can contain water which will cause the stone to fracture when heated.

Iridescence (v) Optical phenomenon describing a material’s apparent surface hue change corresponding to the angle at which the surface is viewed.

Isotropic (a) A singly refractive gemstone such as Diamond.

Kimberlite (Pipe) (n) Kimberlite is an igneous rock composed of olivine, phlogopite, pyroxene and garnet, with a variety of chemically anomalous trace minerals. Kimberlite occurs in the Earth’s crust in vertical structures known as kimberlite pipes.

Lap (n) A grinding wheel or grinding surface used in a faceting machine such as a “Facetron”.

Loupe (n) A small magnifying glass that is hand held or placed in the eye socket. A 10X (10 times or 10 power) loupe should be used to examine gemstones.

Lustre (n) From the Latin word “lux”, meaning “light”. Describes the way light interacts with the surface of a mineral or gemstone.

Macle (n) A twinned, irregular shaped pyramidal crystal. Derived from the Latin word macula, “mesh.”

Mêlée (n) Rough stones and shapes under two carats. Also used to describe small polished diamonds.

Metamorphic Rock (a, n) They are formed deep beneath the Earth’s surface under great stress from high pressures and temperatures. Garnets are a Metamorphic mineral.

Mineralization (n) A process where an organic substance is converted to an inorganic substance. This transformation process can occur in Fossilization.

Mohs scale (a, n) Created, in 1812, by the German mineralogist Friedrich Mohs, the Mohs’ scale of hardness quantifies the scratch resistance of minerals by comparing the ability of a harder material to scratch a softer material ranging from Talc (1) to Diamond (10). Corundum (Ruby or Sapphire) has a hardness of 9 therefore Diamond is 4x harder than Corundum.

Monoclinic (a) In the monoclinic system, the crystal is described by three vectors of unequal length. Two pairs of vectors are perpendicular, while the third pair make an angle other than 90º.

Nacre (n) In Pearls, multiple layers of calcium carbonate that surround a nucleus and giving the pearl a iridescent appearance.

Octahedron (n) A polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles.

Parting (n) A plane of weakness in the crystal structure along planes that are weakened by the application of force.

Pegmatite (n) A coarse-grained igneous rock composed of feldspar mica, and quartz, that is formed deep within the earth’s crust. Pegmatite rock is usually associated with Apitite, Aquamarine, Beryl, Topaz, Tourmaline, and Quartz.

Perthite (n) Describes the intergrowth of two feldspars; potassium-rich alkali feldspar, and lamellae intergrowths of sodic alkali feldspar.

Pinpoint Inclusions (n) Minute light or dark crystals that are visible by themselves or in clusters. Larger clusters of minute pinpoints can create a hazy area in the diamond, called a cloud, affecting clarity.

Pleochroism (n) Color changing effect created by light traveling along crystallographic axes causing the mineral to display more than one color when rotating and/or viewing the stone from different angles.

Polycrystalline (n) A polycrystalline material is made of several smaller crystallites (single crystals).

Polymorphism (n) Meaning “many forms”, a single chemical composition can exist with two or more different crystal structures.

Pseudochromatic (n) False color caused by optical effect such as spectral dispersion (prism effect) or scattering.

Pseudomorphism (n) A mineral that has the appearance of another mineral.

Refractive Index (a, n) As light passes from air into a material it is bent or refracted. The Refractive Index is a measure of the amount that light is bent.

Rutile (n) Rutile derives its name from the Latin word “Rutilus” or “red”, and is used to reference the deep red color observed in some specimens. Rutile is a mineral composed of titanium dioxide and is a tetragonal mineral usually forming in a prismatic or twinned configuration.

Scintillation (n) Used to describe a secondary quality aspect of a gem’s brilliance, scintillation describes sparkle created with movement of the stone. One of the three categories of ‘brilliance’, ‘fire or dispersion’ and ’scintillation’.

Schist (n) Part of the later stage of the “rock cycle” (sedimentary > metamorphic > igneous), schists is a type of metamorphic rock containing lamellar minerals such as micas, chlorite, talc, and graphite, along with prismatic minerals and mineral constituents, such as garnet and tourmaline.

Sedimentary Rock (n) One of the three main rock groups along with igneous and metamorphic rocks, it is formed by compaction, deposition, erosion, and transportation. Sedimentary rocks cover 75% of the Earth’s surface.

Sediment (n) Sediment is any particulate matter that is transported by water and eventually deposited as a layer of solid particles on the bed or bottom of a body of water.

Streak (n) The color which a mineral displays when it has been ground to a fine powder.

Surface Sparkle (n) A diamond’s stationary brilliance, or brilliance without movement. Sharp contrast of dark and light between adjacent facet reflections.

Tenacity (v) The resistance of a mineral to breaking, crushing, or bending.

Tetragonal Crystal (a, n) In crystallography, tetragonal crystal lattices (structures) result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base.

Thermal Shock (a) Fracturing of a gemstone due to rapid temperature change. Moisture content that is trapped within inclusions or fissures can expand and contract due to extreme temperature shifts, causing fracturing.

Toughness (n) Toughness indicates a stone’s resistance to breaking, chipping or cracking when subjected to mechanical stress, impact, or a fall. Toughness is rated as: exceptional, excellent, good, fair, poor.

Trigonal (a) A crystal system is described by 3 basis vectors. In crystallography, the trigonal crystal system is one of the 7 lattice point groups. Quartz is represented as D3 in the Schoenflies notation of Point groups in 3 dimensions.

Twinning (Crystal) (n) Two or more intergrown crystals, formed in a symmetrical arrangement.

Uniaxial Gemstones (a, n) In uniaxial stones, the optic axis is parallel to the c-axis. Biaxial gemstones have two optical axes that do not line up with the c-axis.

Vitreous Lustre (Luster) (a) Describes a highly reflective, nonporous surface similar to that of glass. A “glassy” reflective quality.

Windowing (a) In faceted, colored gemstones, light that is not refracted off of the pavilion and back through the crown and table will pass directly through the stone, appearing like a hole or “window”.

Zoning (n) Angular or straight color banding or “zoning” in natural gemstones and synthetics.