Bone Opal Identification Guide
How to identify bone opal, opalized fossil bone, by its bone structure, hardness, and difference from ordinary opal and petrified wood.
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What Bone Opal Looks Like
Bone opal (also called opalized bone) is fossil bone in which the original bone has been replaced by or impregnated with opal (hydrated silica). It usually preserves the bone's internal structure: a porous, honeycomb-like spongy interior or the tubular Haversian channels of dense bone. Colors range from white, cream, and gray to honey-brown, and the silica may be common opal (no play-of-color) or, rarely, precious opal that flashes spectral colors. Luster is waxy to vitreous, and pieces are translucent to opaque.
Step-by-Step Field ID Checklist
- Look for recognizable bone anatomy: spongy cancellous texture or fine tubular canals.
- Note the outer shape, which may mimic a bone fragment, tooth, or joint.
- Check for a waxy to glassy silica luster on fresh surfaces.
- Tilt the piece in light to see if any precious opal play-of-color appears.
- Test hardness with a steel knife and quartz.
Key Diagnostic Tests
- Mohs hardness: ~5.5–6.5, typical of opal; it scratches glass weakly and is scratched by quartz.
- Streak: white.
- Fracture: conchoidal in the silica; the bone structure may control breakage along porous zones.
- Acid: no fizz with dilute HCl (distinguishes silica-replaced bone from unmineralized or calcite-replaced bone, which fizzes).
- Density: moderate (SG ~2.0–2.2 for opal), lighter than chalcedony-replaced fossils.
- Structure under a lens: preserved bone microstructure is the clinching feature.
Common Look-Alikes and How to Tell Them Apart
- Ordinary common opal or opalite: lacks any internal bone structure or anatomy.
- Petrified (opalized) wood: shows wood grain, growth rings, and cell structure rather than spongy bone or Haversian canals.
- Agatized/chalcedony bone: harder (7) and denser; fills the same pores but with chalcedony, so it scratches glass easily and rings differently.
- Modern or sub-fossil bone: lightweight, may smell when wet, often reacts to acid, and is not silicified.
Where Bone Opal Is Found
Opalized bone comes from sedimentary basins where silica-rich groundwater permeated buried bone. The most famous source is Lightning Ridge and Coober Pedy in Australia, which yield opalized dinosaur and reptile bone, sometimes as precious opal. Common opalized bone also turns up in fossil-bearing badlands and desert basins elsewhere.
Frequently asked questions
How can you tell if it's real bone opal?
Real opalized bone shows preserved bone microstructure (spongy interior or tubular canals), has opal's hardness of about 5.5–6.5 and waxy luster, and does not fizz in acid.
What does bone opal look like?
It looks like a bone fragment turned to stone, with a honeycomb spongy interior or fine tubular channels, in white to honey-brown silica that may occasionally flash opal colors.
Bone opal vs petrified wood: how do I tell them apart?
Petrified wood preserves grain, growth rings, and cell walls, while bone opal preserves bone anatomy such as cancellous spongy texture or Haversian canals.
Is all opalized bone precious opal?
No. Most opalized bone is common opal with no play-of-color; only rare specimens, mainly from Australia, are replaced by precious opal that shows spectral flashes.
Bone Opal identified by the community
Recent Bone Opal specimens identified with Rock Identifier.
