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Section 2 - Observation and Magnification
Gemology Foundation
46
Geology
Post-Graduate
10/20/2015

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Cards

Term
Facet
Definition
a flat face cut and polished on a gemstone
Term
Fashioned, Fashioning
Definition

to shape or cut (and usually polish) the material for adornment or ornamental use

 

includes faceting and carving

Term
Cabochon
Definition
a non-facted gemstone cut which consists of a domes and polished top surface with a flat, unpolished back
Term
Color
Definition
an effect in the brain produced by reaction to visible light that reaches the back of the eye; the reaction triggers messages to the brain and color is 'seen'
Term
Transparent
Definition
material through which an object can be viewed clearly and distinctly
Term
Translucent
Definition
material through which an object can be viewed indistinctly or not at all, but through which light can be transmitted
Term
Opaque
Definition
material that allows no light to be trasnmitted
Term
Luster
Definition
a surface reflection, the brightness and quality of which to some extent depends upon the refractive index and the polish or surface condition of the material
Term
Metallic Luster
Definition
the very high reflective luster shown by metals such as gold and silver, and by gem materials such as polished hematite and pyrite
Term
Adamantine Luster
Definition
a very bright and reflective luster, typically displayed by diamond
Term
Sub-Adamantine Luster
Definition
bright luster, often from gemstones having high refractive indices such as zircon and demantoid garnet
Term
Bright Vitreous Luster
Definition

a glossy luster, equal to or even brighter than highly polished glass

 

Ex) polished ruby and sapphire

Term
Vitreous Luster
Definition

the luster seen in polished glass, and in most transparent gemstones whose refractive indices fall within the middle range of values

 

Ex) emerald, tourmaline

Term
Silky Luster
Definition
the luster of certain fibrous minerals such as gypsum and malachite
Term
Resinous Luster
Definition

the luster shown by certain gem materials that are soft and that have low refractive indices 

 

Ex) amber

Term
Waxy Luster
Definition
the wax-like luster which may be shown, for example, by certain microcrystalline gem materials such as turquoise, and by soft gem materials of organic origin such as jet
Term
Physical Properties
Definition

observable and measurable characteristics of materials relating to internal and external physical energy and forces

 

Ex) optical properties, density, hardness

Term
Optical Properties
Definition

those physical properties relating to behaviour under visible light

 

Ex) transparency, reflection, refraction, color, luster

Term
Chatoyancy
Definition

'cat's-eye effect'

 

an internal reflection effect which appears as a bright band of light across the surface of certain gemstones when cut as cabochons

 

the reflection is from fine inclusions, all parallel to one direction, and is visible when the base of the cabochon is cut in parallel with the direction of the inclusions

Term
Asterism
Definition

a star-like reflection of intersecting band of light which is seen from the surface of certain gems when they cut in a particular crystallographic orientation as cabochons

 

an internal reflection effect caused by reflections from two or more sets of parallel fibrous or channel inclusions

Term
Iridescence
Definition

play-of-color

 

iridescence is a color, or serious of colors produced by interference or diffraction when visible light is reflected or diffracted from thin layers or from the internal structures of gems such as labradorite or precious opal

Term
Fire
Definition

a popular term for the spectral colors displayed by a gemstone with a significant degree of dispersion

 

Ex) diamond

Term
Internal Reflection Effects
Definition
optical effects caused by the reflection of light from features below the surface of a gemstone
Term
Inclusion
Definition

internal visible features within cut or uncut gem materials

 

inclusions may be crystals, cavities, bubbles, fragments, fractures, healed fractures, incipient cleavage, swirls and striations

Term
Single, Monophase Inclusion
Definition
an inclusion of a single liquid or gas or solid
Term
Two-Phase Inclusions
Definition
inclusions made up of a combination of a liquid and gas, or a gas and solid, or a solid and liquid
Term
Three-Phase Inclusions
Definition
inclusions made up on liquid, gas and solid
Term
Loupe
Definition

a magnifying lens, ideally of 10x power for the observation of most gem materials

 

the most important instrument used by gemmologists outside a laboratory 

Term
Bamboo
Definition
actinolite crystal, resembling the shape of bamboo, for example, in emerald
Term
Centipede
Definition
intersecting cleavage fissures in moonstone
Term
Feather or Fingerprint
Definition
a partially-healed fracture or fissure or a group of small particles trapped during the natural growth process or synthesis
Term
Horsetail or Ponytail
Definition
fibrous inclusions, radiating form a nucleus in demantoid garnet
Term
Lily Pads
Definition
partially healed tension cracks around a crystal or cavity in peridot
Term
Needle
Definition
thin ro-like crystal inclusion
Term
Rain
Definition
thin straight tubes orientated parallel to each other in beryl
Term
Silk
Definition
fine rutile inclusions that may cause a hazy appearance in the stone, seen especially in corundum
Term
Snowflake
Definition
witish crystallites on a dark background, especially in obsidian
Term
Sun Spangles
Definition
tension cracks resulting in a radial pattern, for example in amber
Term
Tiger Stripes
Definition
ribbon-like rows of partial healing along twinning planes in quartz
Term
Trichites 
Definition
fine fluid inclusions, often join together in a web; may be two-plase; seen in tourmalines
Term
Bright-field Illumination
Definition

the object is illuminated from directly below, giving a bright field of light

 

used for observing transparent or translucent gem materials

 

 

Term
Dark-field Illumination
Definition
observation of a transparent material against a dark background, with illumination fomr the sides rather than from top or below
Term
Top Illumination
Definition

the object is illuminated from above or obliquely

 

used to observe surface features

Term
Protogenetic Inclusion
Definition
inclusions that existed as mineral particles before the host crystal grew
Term
Syngenetic Inclusion
Definition
inclusions that formed at the same time as the host material
Term
Epigenetic Inclusion
Definition
inclusions that formed after the host material stopped growing
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