Opal

Opal Welo honey comb 25x19x11mm 1807ct Precious 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"