GEMSTONE FIELD GUIDE
Opal
Non-crystalline hydrated silica; mineraloid. Common and precious opal; named varieties require separate description
Polished opal stones described by the source as Australian.
Patches of color appear across the polished surfaces. The photograph does not establish treatment, solid versus assembled construction, or geological origin.
Sevenopal / CC BY-SA 4.0 (via Wikimedia Commons) · Source ↗ · CC BY-SA 4.0 International · resized for the webA verified 3D specimen capture is not available for this entry. No generated stand-in is used.
AT A GLANCE
Start with the evidence.
Separate natural opal, synthetic opal, imitation material and assembled construction. A doublet or triplet may contain real opal, while a convincing play-of-color appearance alone does not identify its material.
- GIA — Assembled opal doublets and triplets ↗
- GIA — Natural, treated, synthetic and imitation opal micro-features (2019) ↗
- Refractive index (RI)
- 1.37–1.47 (GIA broad range); 1.44–1.46 (Australian Museum reference)
- Specific gravity (SG)
- 2.15 (+0.08, −0.90) in GIA; 1.98–2.20 in Australian Museum
- Mohs hardness
- 5–6.5 (GIA); 5.5–6.5 (Australian Museum)
Non-crystalline opal
How to read this value
Structure and water content change physical properties. These are different reference scopes, not a promised reading from every opal. A curved surface may yield only an approximate spot reading. An assembled piece can expose a different material at the tested surface; do not treat its reading as proof of the whole object's identity. Hydrophane opal readily absorbs liquids. Have a gemologist assess liquid compatibility before RI/contact or immersion testing; this is a precaution inferred from its documented absorption.
Published reference values, not an identity threshold
How to read this value
Retain source scopes rather than silently blending their ranges. Matrix and absorbed liquid make a whole-piece result unsuitable for direct comparison.
Scratch-hardness references
These are reference properties, not measurements of the pictured specimen. Reference images are educational—not NumisTrends sale inventory.
See the details and limitations ↓01 · IDENTIFY
Build a profile, not a guess.
Reference values concern the opal material, not a matrix-bearing specimen, assembled stone or imitation as a whole.
Refractive index (RI)1.37–1.47 (GIA broad range); 1.44–1.46 (Australian Museum reference)
Non-crystalline opal
Structure and water content change physical properties. These are different reference scopes, not a promised reading from every opal. A curved surface may yield only an approximate spot reading. An assembled piece can expose a different material at the tested surface; do not treat its reading as proof of the whole object's identity. Hydrophane opal readily absorbs liquids. Have a gemologist assess liquid compatibility before RI/contact or immersion testing; this is a precaution inferred from its documented absorption.
Specific gravity (SG)2.15 (+0.08, −0.90) in GIA; 1.98–2.20 in Australian Museum
Published reference values, not an identity threshold
Retain source scopes rather than silently blending their ranges. Matrix and absorbed liquid make a whole-piece result unsuitable for direct comparison.
Mohs hardness5–6.5 (GIA); 5.5–6.5 (Australian Museum)
Scratch-hardness references
BirefringenceNone intrinsic
Non-crystalline opal
Optic characterSingly refractive / isotropic
CleavageNot applicable as crystallographic cleavage
Non-crystalline material
Editorial interpretation of the documented absence of crystal structure; it does not mean resistance to fracture.
Open a property for its scope and source. RI and specific gravity have no units. Mohs describes scratch resistance—not resistance to breaking. Do not scratch a stone to test it.
What can look similar?
Synthetic opal
Laboratory-made opal can show real play-of-color.
How to investigate. A gemologist assesses microscopic growth features and supporting properties.
Laboratory-grown origin and imitation material are different classifications.
Plastic-rich opal imitation
It can reproduce a convincing play-of-color effect without matching natural opal's material properties.
How to investigate. Combine optical observations and microscopy with an appropriate infrared examination.
Do not burn, heat or scratch the object.
Opal doublet or triplet
Face-up viewing can hide the backing or transparent cap.
How to investigate. Check the complete construction and disclose each assembled layer.
An assemblage can contain genuine opal; that does not make it a solid opal.
02 · TESTS & ULTRAVIOLET
What a test can—and cannot—tell you.
Results depend on the specimen, instrument and method. This is a reference for understanding evidence, not a home authentication or treatment verdict.
UV reaction
- Long-wave UV
- GIA's documented 2.39 ct star opal fluoresced strongly bluish white under longwave UV.
- Short-wave UV
- That same star-opal specimen showed weaker bluish-white shortwave fluorescence.
This is one specimen, not a range for all opal. Plastic imitations can fluoresce too, so glow does not establish natural origin or solid construction.
Whole-object construction examination
Whether the apparent gem is solid opal or includes a backing and transparent cap.
Reference observation. A triplet can combine an opal layer with a quartz or glass dome and a separate backing.
Limit. Identify exposed components separately; a cap reading is not a measurement of the opal layer.
Microscopy and infrared assessment
Whether play-of-color material has synthetic growth features or substantial polymer content.
Reference observation. A studied plastic imitation lacked the familiar columnar pattern of traditional synthetic opal; its infrared spectrum showed a strong polymer contribution.
Limit. Absence of a familiar synthetic pattern does not establish natural origin. Use appropriate non-destructive laboratory methods, not a hot-point test.
Real laboratory examples
Documented cases, not test results of the reference specimens shown on this page.
An imitation without the expected synthetic pattern
GIA's 2018 plastic-rich imitation had convincing play-of-color but lacked the columnar and mesh-like features associated with the traditional synthetic comparison material.
The takeaway. A negative result from one visual checklist is not positive proof of natural origin.
Use a qualified gemologist for identification work. Do not expose eyes or skin to UV sources, or perform scratch, heat, acid or solvent tests on a gemstone. Do not assume that an absent UV reaction rules a material out.
03 · TREATMENTS & CARE
Protect the stone you actually have.
Disclosed treatments, fractures and settings can change which cleaning methods are appropriate.
Avoid heat and sudden temperature changes. GIA recommends gentle warm-soapy-water cleaning for opal; assess treatment and assembly before applying general care advice.
Hydrophane opal readily absorbs liquids. As a conservative precaution, have a gemologist assess contact-liquid and immersion suitability before testing; these notes do not recommend soaking an unknown specimen.
04 · SOURCES & SCOPE
Know where each fact comes from.
Original NumisTrends explanations with primary references. This is not a replacement for gemological training, a laboratory report, or the examination of your own stone.
Properties reviewed: 2026-09-07 · Identification notes reviewed: 2026-09-07
- GIA — Opal ↗
- Australian Museum — Opal ↗
- GIA — Opal Care and Cleaning ↗
- GIA — Micro-features of opal (article content) ↗
- GIA Gems & Gemology — Pleochroism in Faceted Gems: An Introduction ↗
- GIA — Gems and Ultrasonic Cleaners ↗
NumisTrends is independent of GIA, Gem-A and the museums cited. A matching property cannot by itself establish identity, natural versus laboratory-grown origin, treatment, geographic origin, grade or value.