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achromatic
greek chroma = colour, a loupe is termed achromatic if the visual field is completely colourless to the very edges. cheap loupes produce coloured edges through dispersion of light and are unsuitable for grading diamonds.

adular
old german synonym for moonstone, named after a location in the swiss lepontine alps, sometimes also called adula-alps

adularescence
the bluish sheen in moonstone and labradorite

allochromatic
from greek chroma = colour and allo = other
the majority of gemstones are colourless if chemically pure and get their colour from impurities; e.g. pure corundum (AL₂O₃) is colourless, ruby and blue sapphire are coloured by impurites of chromium and titanium and iron respectively.
opposite of idiochromatic

anhedral
poorly formed, external crystal faces not (well) developed, opposite of euhedral

anisotropic
doubly refractive, opposite of isotropic

anomalous double refraction
isotropic (singly refractive) materials such as garnet, synthetic spinel, diamond, amber, glass and opal are often subject to internal strain which causes so-called anomalous double refraction that can easily be detected when the gem is rotated between crossed polarizing filters (polariscope).

  

contrary to anisotropic (doubly refractive) stones, which change from light to dark every 90° upon rotation between crossed polars and to singly refractive stones without internal strain, which remain dark upon rotation, anomalous double refraction causes a patchy, striped or tabby (so called because of the resemblance to the fur of tabby cats) extinction pattern which may move upon rotation but in general has no relation to the orientation of the stone between the crossed polars.

in synthetic spinels of aquamarine colour the internal strain is caused by an excess of alumina. the pattern is distinctly tabby, clearly visible and thus provides a diagnostic, easy to detect feature for identification.

aplantic
a lense corrected against spherical and coma (symmetrical) aberration

asterism
star-effect caused by reflection of light at needle-shaped inclusions, e.g. rutile in corundum, present in large enough numbers and orientated along crystal axes; can be 4-rayed (garnet, diopside, rose quartz, white moonstone e.a.), 6-rayed (corundum, black moonstone) or, in rare cases 12-rayed (corundum); only visible in cabochons under a single light (direct sunlight, spotlight etc.). under diffuse light (overcast skyfluorescent tube, ) or in facetted stones no asterism can be observed.

aventurescence
metallic glitter caused by reflection of light at included mineral platelets e.g. fuchsite in aventurine quartz and hematite in aventurine feldspar (sunstone).

baguette
rectangular stone, mostly step cut

baguette princess
rectangular princess cut stone

birefringence
see double refraction

brilliant cut
round cut with 57 or 58 facets: 32 crown (upper part) facets, 24 pavilion (lower part) facets, 1 table facet and sometimes 1 culet at the tip of the stone. the girdle is not counted as a facet. the standard diamond cut.

briolette
drop- or pearshaped cut, the surface of the stone is entirely covered with triangular facets; round or oval cross section; very expensive cut with higher than average waste

bufftop
stone with facetted pavilion (lower part) and cabochon upper part. the convex upper part produces a loupe effect

bullet
a form of cabochon with an extremely high, sometimes pointed upper part

bull´s head
elongated cut-corner triangel, vaguely resembles a sketched bull´s head

cabochon
stone with flat or slightly concave bottom and domed upper part

calibrated goods
a term used by the gemtrade to denote gemstones cut to standardized shapes and sizes so they can be set in pre-fabricated settings.

carat
abbr. ct, plural cts; unit of weight, 1ct = 0.2g

cat´s eye effect
see chatoyancy

chatoyancy
cat´s eye effect caused by reflection of light at needle-shaped inclusions or growth tubes. only visible in cabochon cuts under a single light source (sunlight, spotlight).

checkerboard cut
cut with no table facet, upper part fully covered with lozenge shaped or square facets

CIBJO
Confédération Internationale de la Bijouterie, Joaillerie, Orfèvrerie, Perles et Pierres or World Jewellery Confederation. founded 1926 as BIBOAH, re-named CIBJO in 1961. the task of this council is to record trade practices and nomenclature and to set trade standards accepted worldwide.

cleavage
the tendency of some gems to split along certain crystal planes

colour change
stones may display different colours under daylight and artificial light; the most prominent colour changing stone is alexandrite, a chrysoberyl variety; the phenomenon also occurs in other gemstones e.g. sapphire

concave cut
also "millenniums cut", with concave grooves in the pavilion (the part of a gemstone that lies beneath the girdle)

crown
the part of a gemstone lying above the girdle

cubic zirconia
"cubic stabilized zirconium oxide", often abbreviated to "CZ", very popular diamond imitation. has nothing in common with natural zircon except for the element of zirconium.

culet
the facet produced by cutting off the tip of the stone to prevent damage, old style, usually not found in modern cuts

cushion
cushion shaped cut

density
see specific gravity

diasterism
when the star is best visible in transmitted light, example: star rose quartz
also see asterism

opposite of epiasterism

dichroism
bi-colouredness, a form of pleochroism

dichroscope
optical instrument for the separation of the different colours of pleochroic gems. makes use of the fact that doubly refractive stones split light into two rays polarized at right angles to each other

dispersion
the fire of gemstones caused by the splitting of light into its spectral colours upon reflection at two surfaces inclined to each other. very prominent in demantoid garnet, diamond, white zirkon e.a.

domed table
the table facet of seal stones is sometimes very slightly domed

double refraction
doubly refractive minerals split lightrays in two upon entry, opposite: single refraction

doublet
a composite stone consisting of two parts; common in opals to make use of opal slices of high quality that would otherwise be too thin. sometimes used to defraud e.g. garnet/blue glass doublets or synth. blue sapphire/colourless sapphire doublets may imitate natural blue sapphire. difficult to detect in closed settings (gas bubbles in cement layer)

elbaite
member of the tourmaline group, forms series with liddicoatite, schorl and dravite; first found 1913 on the italian isle of elba, hence the name. elbaites occur in many colours. most famous and sought-after are the blue, purplish-blue and green copper-bearing elbaites from paraïba, brazil.
the discovery of copper-bearing tourmalines in nigeria and mocambique sparked a heated debate whether these new stones may also be termed paraïba tourmalines.
we of the Vienna Gem Center hold the opinion that only elbaites from paraïba, brazil, should be marketed as paraïba tourmalines. in accordance with leading international authorithies we call copper-bearing elbaites from other localities or stones with uncertain origin "paraïba-type tourmalines".

emerald cut
octagonal step cut, the classic cut for emeralds to avoid any damageable sharp edges

epiasterism
when the star is best visible in reflected light, example: star sapphire
also see asterism
opposite of diasterism

euhedral
well-formed, external crystal faces well developed, opposite of anhedral

eyeclean
when a trained expert cannot see any inclusions with the unaided eye

fancy cut
a non-standardized cut, often follows the outlines of the rough stone to achieve maximum yield

fluorescence
in gemmology fluorescence denotes the optical behaviour of gemstones under UV light, quite often this is a diagnostic feature in gemstone identification

free form
asymmetrical fancy cut, usually follows the outlines of the rough stone to achieve maximum yield

gemmology
american english: gemology; the science of gemstones, part of mineralogy with the handicap of having to identify gemstones non-destructively ;-)

girdle
the junction of upper and lower part of a gem, sometimes divided into many small facets. should always be polished as unpolished girdles distract from brilliance. thick girdles mean excess weight and are difficult to set, razor-sharp girdles are likely to chip during setting

Glass
There are glass imitations of almost any kind of gemstones. With some practice and a good loupe identification poses no big problem, though.

The most important diagnostic features are:

• conchoidal fracture (beware: also seen in quartzes and other gems)
• somewhat blunt facet edges (with signs of wear in older "stones")
• round gas bubbles
• often with flowmarks

With larger specimens a simple test of thermal conductivity can be performed to distinguish glass from crystalline material:

Let the stone in question rest next to a definitely crystalline material for a few minutes so that both can assume the same temperature.

Attention: the test only works with crystalline material. It does not work with e.g. Opal, Coral, Amber, Pearls, Ivory and the like. The stone to be tested as well as the reference stone must be of a minimum size, say about 10mm. Small stones assume body temperature far too quickly during testing.

Take the stones up, one after the other, quickly and touch a heat-sensitive spot like lips, eyelid or earlobe. It is imperative to do this as quickly as possible to prevent the stones from assuming body temperature.

Glass feels distinctly warmer than crystal. The difference is quite pronounced.

If you ask yourself whether there is a difference then either there is none (as stated above, if there is a difference, it´s strong), which means that the speciemn is not glass, or the specimens did not have enough time to assume the same temperature.

The test also works with stones in settings, as long as they are large enough to ensure that you do not touch the metal, which always feels cold.

glyptic
the art or process of carving or engraving gemstones

habachtal
a valley in the austrian alps with a famous emerald mine that reportedly has been known since bronce age. the mine is probably the source of most emeralds used in european jewellery until the discovery of the indian and later the south american deposits. the roman emperor nero is said to have possessed a monocle cut from a habachtal emerald. the largest known specimen weighs 42cts and is kept in the london tower.

habit
the typical crystal appearance of a mineral; e.g. tabular hexagonal, columnar prismatic etc.

idiochromatic
self-coloured, fromm greek idio = peculiar, one´s own and chroma = colour
gemstones in which the colouring elements are constituent parts of the chemical formula. examples: malachite (CuCO₃) and dioptase (H₂CuSiO₄) are coloured green by copper, rhodonite (MnSiO₃) gets its rose-red colour from manganese.
opposite of allochromatic

imitations
natural or arftificial substances that imitate natural gemstones e.g. glas or plastics. contrary to synthetic gems imitations do not have the same physical and optical properties as their natural counterparts.

indigolite
also: indicolite, tradename for blue tourmaline, usually, but not always, elbaite

isotropic
singly refractive, opposite: anisotropic

jardin
french for garden, euphemistically used term to describe the inclusions of emeralds.
clean emeralds are extremely rare. thus inclusions in emeralds are accepted much more than in most other gemstones.

labradorescence
the play of colour seen in labradorite feldspar.
the phenomenon is caused by interference of very slightly out-of-phase light due to reflection from thin surfaces. comparable to the colours seen on a film of oil on a water puddle.

layered stone
consisting of two differently coloured layers of agate. as naturally layered stones are extremely rared most layered stones are glued. used as seal stones for gemstone engravings

lodolite
lodolite is an unofficial tradename for quartzes with mineral inclusions belonging to the chlorite group. depending on the type of chlorite the very attractive inclusions appear in various colours e.g. green, brown, orange, white, pink, purple etc.
the literal translation is "mud stone" (spanish lodo = mud).
we are not sure whether this name refers to the mining location in minas gerais, brazil, or to the visual appearnce of the inclusions.

loupe clean
a term only used for diamonds. a diamond is called loupe clean when a trained expert cannot detect any inclusions with the help of an aplanatic, achromatic 10x loupe. diamonds with inclusions of a size of more than 5 microns must not be called loupe clean (including outer features e.g. "frazzles" at the girdle).

marquise cut
also navette cut, reportedly named after the marquise de pompadour

millenniums cut
see concave cut

mixed cut
upper and lower part of the stone are cut in different styles e.g. upper part brilliant or scissors cut, lower part step cut or 3-4 rows of rectangular facets

mohs hardness
the degree of hardness of minerals according to the scale of hardness published in 1822 by the german mineralogist friedrich mohs.

1	talc	talk
2	gypsum	gips
3	calcite	kalzit
4	fluorspar	flußspat
5	apatite	apatit
6	orthoclase feldspar	orthoklas feldspat
7	quartz	quarz
8	topaz	topas
9	corundum	korund
10	diamond	diamant

navette cut
see marquise cut

opalescence
denotes the orange colour that can be seen in opals when viewed in transmitted light. do not confuse with play of colour.

opaque
not transmitting any light

pavilion
the part of a gemstone lying benath the girdle

pendeloque
drop shaped stone (with crown,pavilion and table facet); do not confuse with briolette or teardrop.

plate
stone plate without girdle facets. mostly glued into settings to produce planar surfaces

play of colour
term to describe the various colours of opal when viewed in reflected light, not to be confused with opalescence

pleochroism
multi-colouredness; an optical phenomenon of some but not all doubly refractive stones. caused by the splitting of light into two rays. these rays take different paths through the gemstone, are absorbed differently and thus of different colours. the human eye usually sees a mixed colour. only in very strongly pleochroic gems like e.g. tourmaline, the different colours can be observed by the unaided eye (see dichroscope) when viewing the stone from different angles.

portuguese cut
a cut with 161 facets in ist classical form, because of the high number of facets the portuguese cut is mostly used with large stones having a high pavilion (lower part)

princess cut
this cut was originally developed for diamond baguettes and squares; a combination of a modified step cut and brilliant cut; very lively

profile cut
pentagonal cut that, when viewed from the top resembles a brilliant cut diamond when viewed from the side

radiant cut
a cut for octagonal stones similar to the emerald cut in outline but with a facet alignement similar to the brilliant cut, developed by the american diamond cutter henry grossbard.

refractive index
abbr. R.I., important feature in gemstone identification, the sine-relation of incident ray and refracted ray (refracted rays in doubly refractive materials).

refractometer
optical instrument for the measurement of the refractive index

rose cut
round, facetted stone without pavilion and 3 to 48 crown (upper part) facets

schiller
see adularescence

scissors cut
mostly found in emerald cut stones, shows crossed, scissor-like crown facets. often encountered in cheap (verrneuil) synthetics.

scratch test
to determine the mohs hardness of minerals, hardly used in gemmology which has to work non-destructively

seal stone
mostly oval or cushion shaped stone with slightly bevelled edge, used for engravings (mongrams, coats of arms etc.). table can be flat or very slightly domed.

semi-translucent
transmitting some light only at thin edges

shapes of cut
the most popular shapes:

single refraction
contrary to doubly refractive materials, singly refractive minerals do not split lightrays in two upon entry

specific gravity
density; the weight of a substance compared to the weight of an equal volume of water at 4°C

square
a square shaped stone, either step- or princess cut

star effect
see asterism

step cut
style of cutting rectangular or octagonal stones with parallel facets arranged like steps of a staircase. the classic octagonal emerald cut is a step cut.

synthetic gems
mostly have the same chemical, physical and optical properties as their natural counterparts. as an example for an exception: synthetic spinels have a slightly higher refractive index than natural spinels.

table facet
the largest facet on the crown of a gemstone

teardrop
drop- or pearshaped stone with smooth surface; round or oval cross section

translucent
transmitting some light

trapiche emerald
a special form of emerald; the spanish word "trapiche" denotes the cogwheel with which sugarcane is crushed. in trapiche emerald inclusions of coal-black shale form a pattern resembling the spokes of a wheel. there are at least four types of trapiche emerald:

• a black-bordered core from which black spokes emanate
• a solid black core with black spokes
• coreless with 6 spokes meeting in the center
• core and spokes consisting of emerald with sandstone in between (reverse trapiche)

the phenomenon was also observed in rubies from myanmar (burma) and vietnam

trichroism
tri-colouredness; a form of pleochroism

trilliant cut
also trillion, a triangular cut with outwardly curved sides. has more or less replaced the classic straight-sided triangel as trilliants are much less susceptible to damage during cutting and setting.

triplet
a composite stone consisting of three parts. most common in opal when a paper-thin layer of opal is glued onto a base of onyx or plastic and covered with a glass- or sometimes rock crystal top

tumbled
cheap gemstones are not polished by hand individually, they are tumble-polished in large quantities in rotating or vibrating tumbling machines which results in irregularly shaped "pebbles".

verneuil, auguste
1856-1913, french chemist who in 1902 published the flame-fusion process for the mass-production of cheap synthetic gemstones. up to today large quantities of synthetic gems, mostly rubies, sapphires and spinels, are produced by this method. contrary to much more expensive synthetic gems produced by other methods, verneuil stones in most cases are relatively easy to identify for gem experts.

typically shaped "verneuil-boule"

zoning
also colour zoning; growth-related patchy or striated colour distribution within a gemstone. common in quartz and sapphire; no longer a sure sign of genuineness as some synthetic gemstones may also display colour zoning

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