Fossil Opal Identification Guide

What Fossil Opal Looks Like

Fossil opal (opalized fossil) is opal — hydrated amorphous silica (SiO2·nH2O) — that has replaced or filled the original organic material of a shell, bone, wood, or other fossil. The shape preserves biology; the substance is opal.

• Color: ranges from milky/white "potch" through honey and grey to vivid precious opal with play-of-color; body follows the original fossil form.
• Luster: waxy to vitreous.
• Transparency: translucent to opaque.
• Form: recognizable fossil shapes — bivalve and gastropod shells, belemnites, wood grain, bone — rendered in opal, sometimes with rainbow flashes.

Step-by-Step Field-ID Checklist

1. Identify the fossil structure first: shell ribs, growth lines, wood grain, or bone texture preserved in three dimensions.
2. Confirm it is opal, not calcite/aragonite: check hardness and the acid test (opal does not fizz).
3. Tilt for play-of-color — precious opalized fossils flash spectral colors.
4. Heft it: opal feels light for its size.
5. Check the silica feel: waxy luster, conchoidal chips.

Key Diagnostic Tests

• Mohs hardness: 5.5–6.5. A knife scratches it; it does not scratch quartz.
• Streak: white.
• Fracture: conchoidal, no cleavage.
• Acid: no reaction to dilute HCl — crucial, because the original shell (calcite/aragonite) would fizz vigorously. No fizz confirms silica replacement.
• Density: low, ~1.9–2.2 g/cm3; noticeably lighter than a calcite fossil of equal size.
• UV: opal often fluoresces greenish-white.

Common Look-Alikes and How to Tell Them Apart

• Calcified / aragonitic fossils: original shell preserved as carbonate fizzes in acid, is softer (3), and shows rhombohedral cleavage. Opalized fossils do not fizz and have conchoidal fracture.
• Agatized / silicified (chalcedony) fossils: also silica, but chalcedony is harder (7) and lacks opal's play-of-color and lower density. If it scratches glass cleanly and is hard, it is agatized, not opalized.
• Pyritized fossils: metallic golden, heavy, and magnetic-adjacent (pyrite is not magnetic but is dense); clearly not silica.
• Plain potch opal: opal without fossil structure; the diagnostic of fossil opal is the preserved biological form.
• Plastic/resin imitations: very light, warm to the touch, soft, may smell when hot-pin tested; lack genuine play-of-color depth.

The identification rests on recognizable fossil form + opal properties (soft ~6, low density, conchoidal, no acid fizz, possible play-of-color).

Where Fossil Opal Is Found

The world's premier opalized fossils come from Lightning Ridge and Coober Pedy, Australia, where Cretaceous shells, bones, and even opalized dinosaur remains occur. Opalized wood is also found in the USA (Nevada, Oregon), and opalized shells turn up in other Australian and some Indonesian deposits.