Joy of Museums Virtual Tours

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Ammonia – American Museum of Natural History

Ammonite - American Museum of Natural History

Giant Fossil Ammonite

This Ammonite shell of a marine animal that went extinct about 65 million years ago. The shell colorization is only found in ammonites from Alberta, Canada.

Originally the ammonite shell was made of aragonite, which is the same material as which pearls are made. Over millions of years, this animal shell was subject to significant temperatures and pressures and was transformed into a gemstone.

It was recognized as a gemstone in 1981 by the World Jewelry Confederation and named Ammolite. Only three types of gems are made by living organisms, this one plus amber and pearls.

The name “ammonite” was inspired by the spiral shape of their fossilized shells, which resemble tightly-coiled rams’ horns.

Pliny the Elder (d. 79 AD) called these fossils “horns of Ammon” because the Egyptian god Ammon was typically depicted wearing ram’s horns. 

Ammonoids are an extinct group of marine mollusk animals and are more closely related to octopuses, squid, and cuttlefish.

In medieval Europe, fossilized ammonites were thought to be petrified coiled snakes, and were called “snakestones” or “serpent stones.”

They were considered to be evidence for miracles by saints and were held to have healing or prophetic powers.

Traders would sometimes carve the head of a snake onto the ammonite fossil or merely paint it on and sell them as petrified snakes.

Ammonites from the Gandaki river in Nepal are known as Saligrams and are believed by Hindus to be a concrete manifestation of God.


Salagrama or Shaligram refers to a fossilized shell used in South Asia as an iconic symbol and reminder of the God Vishnu for some Hindus.

Shaligrams are usually collected from river-beds such as the Gandaki river in Nepal. They are considered easy to carry and popular in certain traditions of Vaishnavism, as a symbolic or suggestive representation of the divine. 

They are typically in the form of spherical, black-colored Ammonoid fossils.


Snakestones, also known as Serpentstones, were fossilized ammonites which were thought to be petrified coiled snakes and were called snakestones.

They were Mythological objects considered to be evidence for the actions of saints, such as Hilda of Whitby, a myth referenced in Sir Walter Scott’s Marmion, and Saint Patrick.

They were held to have healing or oracular powers in Medieval legends. Mythological objects encompass a variety of items found in mythology, legend, folklore, religion, and spirituality from across the world.


Ammonoids are an extinct group of marine mollusk animals, commonly referred to as ammonites. They are more closely related to octopuses, squid, and cuttlefish than they are to shelled Nautilus species. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species is found to specific geologic periods.  The last species died out in the Cretaceous–Paleogene extinction event.

Cretaceous–Paleogene Extinction Event

The Cretaceous–Paleogene extinction event was a sudden mass extinction of some three-quarters of the plant and animal species on Earth, about 66 million years ago.

It is generally thought that the extinction was caused by the impact of a massive comet or asteroid which devastated the global environment, mainly through an impact winter which halted photosynthesis in plants and plankton. 

Other contributing factors to the extinction may have been volcanic eruptions, climate change, and sea-level change.


Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites. 

The following criteria determine the quality of gem ammolite and the value of an ammolite gemstone:

  • The number of primary colors
    • A broad array of colors is displayed in ammolite, including all the spectral colors found in nature. The most valuable grades have three or more primary colors.
  • The way the colors “play.”
    • The Chromatic shift is how the colors vary with the angle of viewing and the angle of light striking the gemstone. The rotational range is how far the specimen can be turned while maintaining its play of color.
  • Brightness of colors
    • The brightness of colors and their radiance is mostly dependent on how well-preserved the nacreous shell is, and how fine and orderly the layers of aragonite are.


How an ammonite becomes a fossil

60 million-year-old Ammonite fossils fully preserved

A Tour of the American Museum of Natural History, New York

Cracking a great fossil Ammonite open

A Tour of Natural History Museums



“Isn’t it astonishing that all these secrets have been preserved for so many years just so we could discover them!”
– Orville Wright


Photo Credit: By Joyofmuseums (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons

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