Shell Opal

Shell Opal is a type of fossil opal in which ancient shells, such as bivalves, gastropods, belemnites, or ammonites, have been replaced or infilled by opal. The opal preserves the detailed shape and ribbing of the original shell.

As hydrated silica it may be common opal showing a solid body tone, or, in the finest examples, precious opal that flashes play-of-color across the fossil shell.

Opalized shells are treasured by collectors for uniting paleontological interest with gem beauty, and Australia produces some of the world's most spectacular examples.

Shell Opal forms when buried marine shells dissolve or leave porous voids in the sediment, and silica-rich groundwater later fills those spaces. The depositing silica hardens into opal in the exact shape of the shell.

If the silica spheres arrange into a regular diffracting lattice, the shell becomes precious opal; otherwise it forms common opal with a steady color.

The opal fields of Australia, including Lightning Ridge, Coober Pedy, and White Cliffs, sit on ancient inland seabeds and yield abundant opalized marine shells. Similar material occurs in other former marine sedimentary basins.

Look for clear shell form, including ribs, whorls, or chambered structure, rendered in opal with a vitreous to waxy luster and white streak. Hardness is 5.5-6.5.

Gem-quality pieces show play-of-color tracing the shell's surface; common-opal shells show only body color.

Distinguish from chalcedony or calcite-replaced shells: chalcedony is harder (around 7) and calcite is much softer (3) and fizzes in acid. Opal's intermediate hardness, water content, and potential color flash are diagnostic.

Shell Opal is prized as both collectible specimens and jewelry. Precious opalized shells are set as rare statement pieces or kept whole for display, while more common material is cut into cabochons.

As soft, hydrous opal it needs careful handling, kept away from heat, chemicals, and dryness that can craze it.

Value depends on the completeness and recognizability of the shell, the brilliance of any play-of-color, and the rarity of the species. Exceptional opalized ammonites and belemnites are highly sought after.