Fossil Opal

Fossil Opal refers to fossils in which the original organic or shelly material has been replaced or infilled by opal. The result preserves the form of ancient wood, shells, bones, or other organisms in hydrated silica.

Depending on conditions, the replacing opal may be common opal (a solid body color) or, in prized cases, precious opal that flashes play-of-color across the fossil shape.

Opalized fossils are scientifically valuable as records of past life and highly collectible when they combine a clear fossil form with gem-quality opal.

Fossil Opal forms when buried organic remains decay or dissolve, leaving a mold or porous structure that silica-rich groundwater later fills. As the silica deposits and hardens, it takes on the shape of the original organism.

Where the silica spheres organize into a regular lattice, the replacement becomes precious opal with play-of-color; otherwise it forms common opal.

Australia is the world's premier source of opalized fossils, including opalized shells, wood, and even dinosaur and marine reptile remains from Lightning Ridge and Coober Pedy. Opalized wood also occurs in the western United States and elsewhere.

Look for recognizable fossil structure, such as wood grain, shell ribbing, or bone texture, rendered in opal with a vitreous to waxy luster and white streak. Hardness is 5.5-6.5.

Gem-quality fossil opal may show play-of-color following the fossil's form; common-opal fossils show steady body color.

Distinguish from agatized or silicified (chalcedony) fossils, which are harder (around 7) and lack opal's water content and potential play-of-color. The combination of fossil shape and lower opal hardness is diagnostic.

Fossil Opal is collected both as scientific and ornamental material. Precious opalized fossils are cut and set as rare jewelry pieces or kept as display specimens; opalized wood is popular for cabochons and decorative slabs.

Because opal is soft and can craze, specimens are stored away from heat and dryness and handled carefully.

Value depends on fossil clarity, the quality of the opal, and the rarity of the organism. Opalized dinosaur and marine reptile fossils can be exceptionally valuable to both collectors and museums.