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Opal

 

Opal Welo honey comb 25x19x11mm 1807ctPrecious opal, 18,07 ct.

 

Name: from Pliny (AD) after greek opallios.

 

Crystallographic characteristics:

amorphous to partially crystalline

Structure: silica gel spheres (150-400 nm diameter), arranged in a cubic densest packing

 

Chemical characteristics:

Chemical formula: SiO2 x nH2O

Types of opal

Precious opal

opal with colourful play-of-colours, opaque to translucent, with dark (black opal) or lighter (light or white opal) body colour; transparent precious opals are called crystal opal

Matrix opal

precious opal in host rock

Boulder opal

matrix opal: Concretion of clay and limonite with thin opal veins

Opal matrix

opal-bearing rock; its matrix can be dyed to dark colours

Fire opal

transparent to translucent, red (incorporation of iron hydroxides), sometimes with play-of-colour

Andes opal

opaque blue to greenish-blue (incorporation of chrysocolla)

Pink opal

opaque pink (incorporation of palygorskite)

Prase opal

opaque green (incorporation of nickel minerals)

Girasol

almost colourless with undulating blue opalescence

Cacholong

opaque to translucent, porcelaneous to milky white with gray, yellowish or reddish hues

Hydrophane

porous, often dull opal, absorbs water

 

Physical characteristics:

hardness

5½ - 6½

fracture

shell-like to brittle

density

1.74 - 2.22 g/cm3
(lower values for hydrophane)

refractive index

n = 1.450 (1.370 - 1.490)

 

Optical effects:

Play-of-colour

Diffraction of white light on the three-dimensional arrangement of silica gel spheres. Subsequent interference creates the play-of-colour with pure spectral colours. Individual colour fields are created through areas of different arrangement and size of the silica gel spheres.

Opalescence

Absorption process by which an opal develops a bluish shimmer when viewed from above, since the blue light is reflected; when viewed through it appears red since the red light is transmitted

Chatoyance (cats-eye effect)

occasionally (chrysotile fibres)

 

Microscopic characteristics: see inclusion gallery

 

Occurrences: a.o.

Australia

white opal, black opal, boulder opal, Yowah-nut opal

Mexico

white and black opal, fire opal

Brazil

white opal, fire opal, opal cats-eyes

Ethiopia

white opal (often hydrophane!), crystal- or water opal,
rarer yellowish to orange fire opals and stones with red and brown body colour

Peru

Andes opal, prase opal, pink opal

other occurrences

a.o. Indonesia, USA, Slovakia, Mali

 

Treatments:

Kind of enhancement Identification Frequency, stability

Dyeing

Blackening of porous matrix opal (Andamooka-matrix) with sugar charcoal process

Dyeing of hydrophane opal with various dyes or "smoke treatments"

sugar charcoal process: dark residues as fine particles
Dying and "smoking": sometimes colour concentration in fissures and patchy colour distribution

Furhter methods:
Infrared- & Raman spectroscopy

common

usually stable (dyed hydrophane opal could bleach after a while)

Impregnation

Improvement of stability

Furhter methods:
Infrared- & Raman spectroscopy

occasionally

stable

Irradiation

Alteration of colour

special fire opals from Brazil change in colour due to irradiation with electrons to blue or green-blue

rare

stable

 

Synthetics:

Production method Identification Specifics

Sedimentation from a mono-disperse silica gel suspension

all colours

exact division of colour fields
"lizard-skin" effect
columnar structure when viewed from the side

Further methods:
Infrared spectroscopy

occasionally lower density (1.63 - 1.90 g/cm3)

 

Composite opals and imitations:

Types Identification Specifics

Dublets and triplets

all colours, sometimes with synthetic opal

dublets: clear boundary between the crown made of precious opal and the pavilion made of glass / onyx / sandstone / etc

triplets: additionally a cover made of a resistant stone

opal-mosaic-triplets: small pieces of opal on an dark background, embedded in artificial resin

Artificial glasses and plastics

different physical characteristics (refractive index, density)
different microscopic characteristics (see inclusion galery)

"Slocum" or "Pastoral"

 

 

 

Copyright

Editorial board: Dr. rer. nat. Tom Stephan, M.Sc., F.G.G., E.G. (responsible) and Dr. rer. nat. Ulrich Henn, Dipl.-Min., assisted by Qi Wang, M.F.A., F.G.G., E.G.

The German Gemmological Association (DGemG) reserves all rights for all contributions. The contributions are exclusively for private use. Any further use of texts and images for non-private use requires the written permission of the German Gemmological Association (DGemG).