August 23, 2016

Glossary

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Glossary of Terms

acicular
needle-shaped, slender like a needle.

acute bisectrix
in optically biaxial minerals, the direction bisecting the acute angle between optic axes.

adamantine
a type of lustre suggesting extreme hardness or high refractive index such as in diamond; from” adamas”.

amorphous
without form; applied to solids having no definite crystalline structure.

angle of incidence
in optics, the angle between the incident ray of light and the normal to the surface.

anion
an ion that bears a negative charge.

anisotropic
in optics, showing birefringence.

anode
electropositive pole.

anomalous double refraction
the appearance of birefringence in isotropic material caused by internal strain; observable between crossed polars.

asterism
a star-like reflection from oriented fibres or fibrous cavities in a stone cut in a properly oriented cabochon shape, having several sets of rays crossing at angles related to the mineral’s crystal system.

axis (crystallographic)
one of the imaginary lines used for reference in a crystal, or a defined direction in crystalline material.

axis of symmetry
An imaginary line through the centre of a crystal, about which the crystal may be rotated to show the same appearance, two, three, four or six times in one complete rotation.

basal cleavage
cleavage which occurs parallel to the basal pinacoid of a crystal

basal pinacoid
a crystal form consisting of only two parallel faces, so oriented as to cut the vertical c-axis and to be parallel to the plane of the lateral axes.

biaxial
in optics, a crystal with two optic axes and three indices of refraction.

bipyramid
a crystallographic form consisting of two pyramids across a plane of symmetry.
It is useful to use the prefix bi to denote that a form consists of two portions repeated over a plane of symmetry, and the prefix bi to denote repetition of pairs of faces around an axis.

birefringence
the numerical difference between the greatest and least indices of refraction; see double refraction.

bisectrix
a line bisecting the angle between the two optic axes of a biaxial crystal, designated acute or obtuse, depending on which of the supplementary angles is being bisected.

botryoidal
a habit of crystal growth resembling the shape of a bunch of grapes, e.g. smithsonite.

brefke
The small paper envelope that diamond dealers use to carry around diamonds.  From Flemish.

brittleness
the property or “tenacity” of material that ruptures easily.

cabochon
a cut and polished gemstone having a domed or convex shape, without facets.

Canada balsam
a resin obtained from a species of fir; used as an adhesive in optical instruments; RI. 1.524, cooked, 1.534-1.540.

carat
a unit of weight. The international metric carat is 200 milligrams (1/5th of a gram). The abbreviation of carat or carats is simply “ct”, for singular and plural, with no period.

Carlsbad twin
a type of crystal twinning common in feldspars.

cleavage
the tendency of a crystalline mineral to break along certain definite directions producing more or less smooth, flat surfaces.

closed form
a crystal form that encloses a finite volume of space.

columnar
a crystal habit composed of a series of slender prisms. If the columns are very fine, the habit is called fibrous.

composition plane
a plane by which the two individuals of a contact twin are joined.

conchoidal
a type of fracture giving smoothly curved surfaces resembling a shell, e. g. quartz, glass, plastic.

contact twin
where the twin halves of a crystal have grown with one-half rotated 1800 to the other. In diamond, a contact twin is called a “made”. Also typical of spinel.

critical angle
in optics, the angle of incidence at which the refracted ray is 900, thus just grazing the surface contact between two media. Any increase in the angle of incidence produces “total internal reflection”.

crossed polars 
two Nicol prisms (or “polars”, which today are usually polaroid filters) placed so that their vibration planes are mutually at right angles.

crown.
The portion of a faceted gemstone between the girdle and the table.

cryptocrystalline
referring to a rock or mineral consisting of crystals that are too small to be seen except with an electron microscope or at high magnification of a thin section between crossed polars.

crystal
a solid possessing a definite internal atomic structure, which is identical for all crystals of anyone species, and which may be reflected in a definite, external, geometric shape bounded by plane faces.

crystal face
one of a number of more or less flat surfaces bounding a crystal.

crystal lattice
the regular and repeated three-dimensional arrangement of atoms or ions in a crystal.

crystalline
any material which shows by physical and optical properties the regular arrangement of its internal atoms. In contrast to amorphous.

crystal system
one of seven basic groups into which all crystals can be classified according to their symmetry.

culet
the facet or point at the bottom center of a faceted gem.

cutlet
a serving of pork.  It has nothing to do with jewelry.

cyclic twinning
repeated twinning of three or more individuals according to a twin law with the twinning axes not parallel. These often simulate high symmetry (four-, five-, six,-or eight-fold axes), some of which are impossible in an untwinned crystal, e.g. chrysoberyl “trilling”, a cyclically twinned crystal which is pseudohexagonal.

dendritic
a branching or tree-like feature, typical of the type of inclusion seen in moss agate

diffraction
a term applied to the bending of light rays not in accord with Snell’s law-for example, the bending of rays around obstacles; the breaking up of white light into spectral colours when it passes through a narrow aperture.

diffraction grating
a series of extremely close and evenly spaced lines which cause light to diffract producing a spectrum.

dispersion
the power of a transparent medium to separate white light into the spectral colours. Measured by the difference between its RI. for red light and for violet light, commonly using the B-G interval of the spectrum.

double refraction
a property of anisotropic crystals, which split a ray of ordinary light into two polarized rays which traverse the crystal at different speeds and in different directions. See birefringence.
dome
a crystal form whose faces intersect the vertical crystallographic axis and one horizontal axis, but are parallel to the third axis.

 

extraordinary ray
that ray in a doubly refracting, uniaxial mineral in which the velocity varies according to the direction in which it passes through the crystal.

facet
the polished plane or flat surface of a cut gemstone.

form
a group of crystal faces that includes all those similarly related to the crystallographic axes. The assemblage of all faces required by the symmetry when one face is given.

fluorescence
visible light produced by a substance exposed to light or other electromagnetic radiation of shorter wavelength (e.g.ultraviolet light).

fracture
the manner of breaking and appearance of a solid when broken, producing other than plane surfaces.

grain
in diamond, the direction across the edges of octahedral faces.

habit
the characteristic external shape commonly adopted by a crystalline material.

hackly
showing jagged points in a fracture.

hardness
the power a substance possesses to resist abrasion (or scratching) when a pointed fragment of another substance is drawn across it.

hearts & arrows (H&A)
A pattern in the symmetry of diamonds visible through a special viewer.

hemimorphic
having no transverse plane of symmetry and no centre of symmetry. The ends of hemimorphic crystals have different terminations.

holohedral
referring to the crystal class showing the full symmetry of a crystal system.

homogeneous
of the same kind or nature throughout.

igneous rocks
the product of the crystallization of magmas; may be volcanic (extrusive), or plutonic (intrusive); includes all rocks of magmatic and plutonic origin

interference
in optics, when two rays of white light travelling in the same direction but out of phase interfere with one another, they may cause either total extinction of light or strengthening of one or more colours.

ion
an electrically charged atom, radical or molecule.

isometric
cubic; same measurement in like directions.

isomorphous
having the same shape; minerals of different chemical compositon may have similar atomic structures with differing atoms in equivalent positions, e.g. calcite, rhodochrosite, siderite. All commonly crystallize in rhombs.

isomorphous replacement
the replacement in a crystal structure of one element with another element having the same valency, and (in a solid solution series) a similar size, and which, without sensibly altering the crystal structure, may cause wide variations in physical properties, e.g. garnet, tourmaline, feldspar.

isotropic
having the same optical properties in all directions, and only one index of refraction.

lamellar
in thin plates or layers.

magma
hot silicate melt, the parent material of igneous rocks.

massive
without definite external crystal shape; may be amorphous or crystalline.

melee
Small diamonds and gemstones used as decorations in jewelry.  Usually under 0.10cts each.

metamict
describing a mineral containing a radioactive element in which the crystal lattice has been disrupted by radiation. At the same time the original external shape is retained.

metamorphism
the transformation of pre-existing rocks into new types by the action of heat, pressure, stress, and chemically active migrating fluids.

metamorphic rocks
rocks which have recrystallized in the solid state in response to pronounced changes of temperature, pressure and chemical environment.

microcrystalline
applied to a mineral in which the individual crystals can only be seen as such under the microscope.

mineral
a homogeneous substance produced by the processes of inorganic nature having a chemical composition, crystal structure and physical properties which are constant within narrow limits.

negative crystal
a cavity in a crystal having the crystal form of the host mineral. It is one type of inclusion.

normal
an imaginary line perpendicular to a surface or perpendicular to a tangent at the surface.

opalescence
a reflection of milky or pearly light from the interior of a mineral; sometimes used incorrectly for play of colour.

open form
a crystal form whose faces do not enclose space, for example, a trigonal prism.

optic axis
a direction of single refraction in a doubly refracting crystal.

origin
the point defined by the intersection of the crystallographic axes.

parallel growth
crystals that have grown together with faces in the same orientation (unlike twinned crystals).

parcel
A collection of gems.  Used in the trading.

pavilion
The portion of a faceted gemstone between the girdle and the culet.

pedion
crystal form with only one face.

phosphorescence
luminescence caused by exposure to ultraviolet or other light or radiation, lasting after exposure has ceased.

pinacoid
an open crystal form consisting of two parallel faces which are parallel to two crystallographic axes and are cut by the third. In a basal pinacoid, the faces are parallel to the lateral axes.

prism
an open crystal form whose faces are parallel to the principal crystallographic axis and cut by the lateral axes.

plane of symmetry
an imaginary plane which divides a crystal into two parts so that each is the mirror image of the other.
play of colour (in opal)
a diffraction phenomenon arising from a three-dimensional structure of minute, uniformly sized, closely packed spheres of amorphous silica.

polysynthetic (twinning)
contact twin crystals producing a number of very thin plates (lamellae). Each adjacent plate is in reverse order (180); alternate plates are in the same order. Also called repeated or lamellar twinning.

prismatic
a crystal habit with elongated straight-sided crystals.

pseudomorph
a crystal having the outward shape of another species of mineral, which has been replaced by substitution or b~ chemical alteration.

pseudosymmetry
apparent symmetry that is different from that characteristic of the mineral, generally the result of twinning, e.g. chrysoberyl II trilling ” is pseudo-hexagonal.

reflection
the turning back of a ray of light which hits a polished surface; the angle of incidence is equal to the angle of reflection.

refraction
the bending of a ray of light as it passes from one medium into another of different optical density.

schiller
silvery reflection or iridescence seen just below the surface of a stone.

sedimentary rocks
formed directly or indirectly from the decay of other rocks, as simple aggregates of particles or as accumulations of mineral and organic materials subjected to a modest degree of chemical change.

solid solution series
see isomorphous replacement; the substitution in a crystal structure of one element for another having the same valency and a similar size, and which, without sensibly altering the crystal structure, causes variations in physical properties, e.g. plagioclase feldspar, topaz.

streak
the colour of the powder of a mineral, obtained by rubbing it on a piece of unglazed porcelain.

striae
the thread-like, roughly parallel lines seen as 1/ curved striae” in synthetic stones, as whorl lines in glass, or as straight lines seen inside natural gemstones.

striations
growth lines on the surface of a crystal.

tabular
referring to a crystal habit in the shape of a book or tablet.

tenacity
the cohesiveness of a mineral-its resistance to crushing or breaking; described as brittle or tough. Tenacity is not necessarily related to hardness.

uniaxial
in optics, a crystal with one optic axis and two indices of refraction.

vug
a cavity in rock, often with a lining of a mineral of a different composition from that of the surrounding rock.

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