Andamooka Opal Identification Guide
How to identify Andamooka opal from South Australia, including matrix opal and the treated 'painted lady' material, and tell it from other opal fields.
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What Andamooka Opal Looks Like
Andamooka is a famous South Australian opal field producing several styles of precious opal (hydrated silica, SiO2·nH2O). You will encounter crystal and light opal with play-of-color on a translucent base, matrix opal where color flecks are dispersed through a porous host rock, and the well-known treated matrix ("painted lady") material, which is darkened by sugar/acid treatment to make play-of-color stand out against a black background.
Play-of-color shows flashes of spectral colors that shift with viewing angle. Body tone ranges from white/light to the dark backgrounds of treated matrix. Luster is waxy to vitreous, and the stone is amorphous (no crystals).
Step-by-Step Field-ID Checklist
- Confirm opal first — look for play-of-color (color flashes that move as you tilt the stone), a waxy-glassy luster, and no crystal faces or cleavage.
- Assess body type — is it solid translucent crystal opal, or color specks in a sandstone-like matrix (matrix opal)?
- Check for treatment — uniformly black background with bright pinpoint color in a porous matrix suggests treated "painted lady" material.
- Test hardness — Mohs 5.5–6.5; opal is scratched by quartz and a hard knife.
- Note porosity — matrix opal is porous and lighter in feel than solid opal.
Key Diagnostic Tests
- Mohs hardness: 5.5–6.5 — softer than quartz.
- Streak: white.
- Cleavage/fracture: none; conchoidal fracture, often with no flat faces.
- Specific gravity: ~1.9–2.2 — notably low; opal feels light for its size.
- No acid reaction (untreated), non-magnetic.
Common Look-Alikes
- Coober Pedy / other Australian light opal: mineralogically identical; Andamooka is distinguished mainly by provenance and its characteristic matrix and treated material, not by a physical test.
- Ethiopian (hydrophane) opal: tends to absorb water and change transparency when wet; most Australian opal does not.
- Opalite / opalized glass: man-made glass shows a milky sheen (opalescence) but no true pinpoint play-of-color and contains bubbles.
- Lab-created opal: shows very regular, columnar color patches ("lizard-skin" or snakeskin pattern) under magnification, unlike the irregular natural play.
Where It Is Found
Andamooka opal comes from the Andamooka opal field in South Australia, one of the country's historic fields alongside Coober Pedy and Mintabie. The opal occurs in weathered Cretaceous sedimentary rocks, filling cavities and impregnating porous matrix host stone.
Frequently asked questions
How can you tell if Andamooka opal is real?
Real opal shows genuine play-of-color that shifts with angle, has a low specific gravity (~2), is Mohs 5.5–6.5, and breaks with conchoidal fracture. Lab opal shows overly regular snakeskin color columns under magnification; glass imitations have bubbles and no true color play.
What is Andamooka matrix opal?
It is opal where the color is dispersed through a porous host rock rather than forming a solid seam. Much of it is treated (sugar/acid darkened) to create a black background that intensifies the play-of-color, known as 'painted lady' opal.
Is Andamooka opal treated?
Some is natural crystal or light opal, but a significant amount of Andamooka matrix opal is treated to darken the background. Reputable sellers disclose treatment; uniform black porous matrix with bright color is a sign of treatment.
How is Andamooka opal different from Coober Pedy opal?
Both are South Australian light/crystal opal and are mineralogically the same. Andamooka is especially known for its matrix opal and treated 'painted lady' stones, so origin and material type, not a physical test, separate them.