Opalized Wood or Shell Fragment (Jewelry Specimen)
Indeterminate (processed as a gemstone); likely fossil wood or bivalve fragment based on internal play-of-color texture • Body fossil / Replacement (Mineralized replacement of biological matter by precious opal)
Geological Period
Cretaceous (likely Early Cretaceous to Late Cretaceous)
Estimated Age
100 - 110 million years ago
Preservation Type
Opalization (replacement of original organic matter by hydrated amorphous silica/precious opal)
Condition Assessment
Excellent (Commercial Grade); the fossil structure is completely mineralized and high-polished for jewelry use
Taxonomic Classification
Kingdom: Plantae (if wood) or Phylum: Mollusca (if shell fragment); Order/Genus/Species indeterminate due to lapidary processing
Morphological Description
Smooth, cabochon-cut oval with intense deep blue and violet play-of-color; subtle underlying grain or layer structure visible through the gemstone surface
Rock Matrix
None visible (removed for jewelry setting); likely originally host in ironstone (boulder opal) or sandstone
Formation & Location
Likely the Winton Formation (Queensland) or Coober Pedy/White Cliffs (South Australia)
Size & Dimensions
Approximately 15mm x 12mm; consistent with standard commercial gemstone cabochons
Value & Rarity
Estimated Market Value
$400 - $1,200 USD (value is primarily determined by opal quality and gold setting rather than fossil rarity)
Auction Estimate
$500 - $900 USD at a specialized natural history or jewelry auction
Rarity Assessment
Uncommon; while opalized fossils are frequent in Australia, gem-quality opal replacement of fossil structures is highly sought after
Scientific Significance
Moderate; provides evidence of fossilization processes involving silica-rich fluids in the Great Artesian Basin of Australia during the Cretaceous period
Preparation Recommendations
Do not use ultrasonic cleaners; clean only with soft damp cloth; store away from high heat to prevent crazing (internal cracking)
Notable Features
Vibrant deep 'electric blue' play-of-color across the entire specimen; the organic origin is reflected in the subtle directional flash of the opal