Orthoceras Polished Egg

Orthoceras Fossil is soft carbonate rock (about Mohs 3) that reacts with acids. Keep it away from vinegar, citrus and acidic cleaners, do not soak it or use salt, and store it apart from harder stones so it does not scratch.

Orthoceras means straight horn. It was a squid-like animal that lived inside a long, straight, cone-shaped shell and hunted in warm Paleozoic seas. When one died, its shell sank into lime-rich sediment and was buried. Over time the original shell was replaced and its empty chambers filled with clear to cream calcite, while the sediment hardened into limestone. The polished pieces sold today mostly come from Morocco, where dark limestone from the Anti-Atlas region near Erfoud is quarried, cut and polished to reveal the shell in cross-section.

Orthoceras is an extinct nautiloid cephalopod. Strictly, the genus name belongs to Middle Ordovician species of Sweden and the Baltic states, roughly 460 million years old. The polished Moroccan pieces are related straight-shelled nautiloids of Silurian to Devonian age, sold under the Orthoceras name. The shell is divided into chambers by walls called septa and pierced by a central tube, the siphuncle. The shells were originally aragonite and are now almost entirely recrystallized calcite. Orthoceras is often confused with ammonites, which are a separate lineage with mostly coiled shells and more intricate chamber walls.

• On the Swedish island of Öland, where Ordovician limestone is rich in straight-shelled nautiloids, these fossils were traditionally called Ölandsspikar, meaning Öland spikes or Öland nails.
• Around 1684, Sir John Clerk of Penicuik set a slab of Swedish orthoceratite limestone into the mausoleum he built for his wife in Scotland, apparently to echo the Roman use of ornamental marbles.
• In Erfoud, Morocco, at the edge of the Sahara, artisans still cut and polish fossil-bearing limestone from a seabed that lay beneath what is now desert, shaping it into slabs, bowls, fountains and tables.