Chapter 2 Observation and Magnification

Observation and Magnification

faceted

facets are cut and polished as a number of flat faces on the surface of the gemstone

most transparent material (coloured and non-coloured gemstones)

name the diamonds top, bottom etc.

top = table crown girdle etc

cabochon

Consist of a domed and polished top surface with a flat, unpolished back.

The outline is round or oval

it flatters the appearance of dark stones or those lacking in transparency

lustre

Lustre relates to the quality and quantity of light reflected from the surface of a material.

It can be described as shiny, dull, or glassy, influencing the appearance of gemstones.

results from a combination of factors, including the chemical and structural nature of the material AND quality of polish

non-faceted stones can be

cabochons, carvings, beads, cameos (on top of a stone)

Optical effect

Chatoyancy

Asterism

Iridenscence

Fire

What is chatoyancy also called?

Cat's eye effect

What is chatoyancy?

The effect is due to reflection of light from regularly orientated inclusions or channels within the gem

which gems can have chatoyancy

Chrysoberyl

Rose quartz

Tourmaline

What is asterism

star effect;

at the surface of certain gems

when they are cut as cabochons

the effect is due to the reflection of light from regularly oriented inclusions or channels within the gem.

The plane of inclusions is parallel to the base.

What is asterism?

An optical effect caused by light reflecting off two or more sets of parallel needle

WHEN cut as a cabochon

which gems can have asterism

ruby

sapphire

garnet

quartz

when cut as a cabochon!

iridescence

play of colour = colour effect

(in greek iris = rainbow)

what is iridescence

The colour effect is seen when light travels through or is reflected or refracted by the surface of very thin layers or cracks in a material, creating a spectrum of colours.

Certain colours in the reflected light interfere and cancel out to leave residual colours

which gems can have iridescence

Opal

some feldspar

Labradorite

Fire

flashes of colour

what is fire

Fire is seen when a transparent faceted gemstone is rotated under a lamp or in sunlight

describe the amount of perceived dispersion in a facetted gem material

which gems can have fire

strongest seen with faceted gems which are pale/colourless = diamond or synthetic moissanite

three types of illumination

bright-field

dark-field

top illumination

Top illumination

illumination from above or obliquely (schuin)

Observation use:

= Observe features on the surface of Opaque/nearly opaque materials

= Observe features within transparent /translucent gemstones

= HELP with detecting differences in lustre on composite or treated stones

bright-field illumination

illumination from directly below

Observation-use:

= transparent + translucent gem material

= certain features on/inside transparent materials

= Not the best for a good examination of the stone

dark-field illumination

illuminated from the sides

not directly from below

Observation use:

Transparent gems - particularly their internal features

what contributes to the beauty of a gem

shape or style of cut

Colour

transparency

Lustre

optical effects

different terms of lustre

@Mettalic:

polished silver and gold

pyrite and hematite

@Adamantine:

diamond

@sub-adamantine

zircon and demantoid garnet

@Bright-vitreous

ruby and sapphire

even to or even brighter than polished glass

@vitrious

the lustre is seen from ordinary windows and bottle glass

It is seen in many rough and polished gems, including emerald, agate and tourmaline.

Dull, earthy, resinous, silky, pearly and waxy other words to describe lustre

Transparency

Transparent: you can see through

Translucent; disrupts the passage of light. some light will pass through

Opaque: allow no light to pass through (hematite, malachite, rhodochrosite)

Internal features can be

@ solid, liquid or gas inclusions:

single mnonophase inclusions

two-phase inclusions

three-phase inclusions

@ zones or crystal growth or other structural features, including colour zones and layers of inclusions.

@inclusions in characteristic shapes such as hollow growth tubes and needle-shaped crystals

@ healed fractures (fingerprints)

twin-plane revealing change in the direction of crystal structure

internal fissures, celavage and stress cracks

evidence of treatment in gems f.x dying, drilling holes, facture filling (ruby, sapphire, emerald)

weight of gems

1 ct = 0.2 gram

5 ct = 1.0 gram

what means achromatic

no colour fringes (randen/franjes)

what means aplanatic

no distortion (vervorming)

Fluorescent table lamp

used for transparent to translucent stones

those lamps with white light tubes; diamond grading

effects as asterism and chatoyancy best seen under a single spotlight