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All About Gemstones -Gemological terms Category: Sciences > Gemology Date & country: 22/08/2013, UK Words: 82

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

Adularescence
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 moonstone gemstones.

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

Allotrope
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 mineral or gemstone that is transported and redeposited by erosion, or running river water. May be redeposited in sediment, sand, or gravel (alluvium).

Amorphous
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, Anisotropism
Exhibiting properties with different values when measured in different crystallographic directions, as with the Mohs hardness rating in kyanite, differing refractive indices in zircon, or differing color in tourmaline..

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

Beryl
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
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
In a anisotropic or doubly refractive mineral, birefringence is the difference in values between the highest refractive indices and lowest refractive indices; causing incident light to be split into two rays.

Brazil Law Twins
In quartz, a Brazil-law twin has right and a left hand structural characteristics which are combined into a single crystal.

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

Brilliant Cut
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
Cutting the rough to the round girdle outline on a brilliant cut.

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

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

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

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

Conchoidal Fracture
Describing the way a brittle material separates or breaks when the fracture does not follow any natural plane of separation.

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

Cryptocrystalline
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
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
Habit refers to the way in which a crystal's atomic structure determines the external shape of the crystal.

Dauphin Law Twins
In quartz, a Dauphin-law twin has either a right or left-handed structure only which is combined into a single crystal.

Detritus (Detrital)
Particles or fragments of rock derived from pre-existing rock through the process of weathering and/or erosion.

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

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

Druzes (Druzi)
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
Residual old-vein deposits of minerals and/or rock that are exposed, or transported and re-destributed by erosion, weathering, or wind.

Enantiomorphic
A pair of crystals which are mirror images of each other, but are not identical.

Epimorph
Crystal growth occurring on the surface of another mineral.

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

Fancy cut
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.

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

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

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

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

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

Gemmy
A stone that glitters like a gem.

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

Girdle
The flattened edge of a diamond or precious gemstone.

idiochromatism
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
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.

In situ
Latin phrase meaning 'in the place.' Describing a gem's physical appearance and primary location prior to weathering and/or transport by erosion.

Inclusion
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
Optical phenomenon describing a material's apparent surface hue change corresponding to the angle at which the surface is viewed.

Isotropic
A singly refractive gemstone such as Diamond.

Kimberlite (Pipe)
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.

Loupe
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.

Metamictization, Metamict
Natural process resulting in a gradual, complete destruction of a mineral,s crystal structure, leaving the mineral amorphous.

Metamorphic Rock
They are formed deep beneath the Earth's surface under great stress from high pressures and temperatures. Garnets are a Metamorphic mineral.

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

Mohs scale
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
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 90

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

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

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

Pegmatite
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.

Peridotite
A dense, ultramafic and ultrabasic, coarse-grained igneous rock, consisting mostly of magnesium-rich olivine, pyroxene, and iron.

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

Petrologic, Petrology
Pertaining to the origin, composition, structure, and alteration of rocks and minerals.

Pinpoint Inclusions
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
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
A polycrystalline material is made of several smaller crystallites (single crystals).

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

Refractive Index
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.

Scintillation
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'.

Sediment
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.

Sedimentary Rock
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.

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

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

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

Tetragonal Crystal
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
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
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 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)
Two or more intergrown crystals, formed in a symmetrical arrangement.

Uniaxial Gemstones
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.

M
Rough stones and shapes under two carats. Also used to describe small polished diamonds.