Nice curves.
Ammonite, Meaning, Properties, and Facts
Fossil · CaCO3Mohs 3 to 4Cretaceous and older
An ammonite is the fossil shell of an extinct sea animal related to squid and octopus. Below: what it is, where it is found, rainbow ammonite, how to spot a fake, how to care for it and the meaning people give it.
What is ammonite
Ammonite fossils are the shells of cephalopods, the group that includes squid, octopus and the nautilus. The animal lived in the newest, largest chamber of its coiled shell and used the older, sealed chambers as a float. The group appeared about 400 million years ago and died out 66 million years ago, in the same extinction that ended the non-bird dinosaurs. Like amber, ammonite is a fossil, not a mineral, so "ammonite stone" and "ammonite crystal" are shop names, not science.
Ammonite properties
Ammonite properties depend on what replaced the shell. The animal built it from aragonite, a form of calcium carbonate, and over millions of years groundwater swapped much of that for calcite, pyrite or other minerals. That is why one ammonite is soft and creamy and the next is heavy and metallic (the Smithsonian's fossil ammonite page covers the animal itself):
| What it is | Fossil shell of an extinct marine cephalopod |
|---|---|
| Age | About 400 to 66 million years old |
| Chemistry | Aragonite shell (CaCO3), often replaced by calcite or pyrite |
| Hardness | 3 to 4 on the Mohs scale; pyritized shells about 6 |
| Crystal system | None: a fossil, not a single crystal |
| Luster | Dull to pearly; rainbow where nacre survives |
| Main localities | Madagascar, Morocco, England, Germany, Russia, Alberta (Canada) and the US Great Plains |
| Water safe | Quick rinse only; never soak |
| Sun safe | Yes |
Where ammonite is found
Ammonite is found worldwide in marine rocks, anywhere an old sea floor of the right age is now dry land. Big sources include Madagascar, the origin of many polished halves; Morocco; the Jurassic Coast of southern England, where Mary Anning collected in the early 1800s; Whitby in Yorkshire; southern Germany; Russia; Alberta, Canada; and the Pierre Shale, mud from the inland sea that once covered the Great Plains. Ammonites even turn up in Antarctica.
Rainbow ammonite and ammolite
Rainbow ammonite and ammolite get their color from the same thing: original nacre, the mother-of-pearl layer of the shell, which splits light into colors the way a soap film does. Many small whole ammonites from Madagascar show patches of it. Ammolite is the strongest version, a gem cut mainly from two ammonite species in the Bearpaw Formation of Alberta, often sold as thin slices backed or capped as doublets and triplets.
How to tell if ammonite is real
How to tell if ammonite is real comes down to sutures and seams. Real ammonites show suture lines, the wavy, branching lines where each chamber wall met the outer shell, repeating in a steady but slightly irregular rhythm. Fakes include resin casts (light, warm to the touch, sometimes with bubbles), carved stone with painted sutures, and real fossils improved with filler, glue or paint, a known problem with some Moroccan fossils. Look for joins, halves that match too perfectly, and color that sits on the surface instead of in the stone.
Ammonite care: water and sun
Ammonite care is mostly about water. Calcite and aragonite shells are soft, and acids such as vinegar or lemon juice etch them; pyritized ammonites can rust and crumble in damp air, a slow breakdown collectors call pyrite decay. Dust with a soft brush, wipe with a barely damp cloth, dry at once and never soak. Sun will not hurt a natural ammonite. See which stones can get wet in water safe crystals.
Ammonite meaning
Ammonite meaning, in crystal lore, is growth and change, read from a spiral that widens with every new chamber. That is belief, not chemistry, and the same goes for claimed ammonite benefits. The history is better: the name comes from Ammon, the Egyptian god shown with ram's horns, and Pliny the Elder called the fossils horns of Ammon. In Whitby, legend said the abbess Saint Hilda turned snakes to stone, and sellers carved snake heads onto ammonites to prove it. In Nepal and India, ammonites from the Kali Gandaki river are revered as shaligram stones, a form of Vishnu.
Ammonite price and value
Ammonite price and value follow size, completeness, preparation and color. Small whole ammonites are common and inexpensive; polished pairs, crisp sutures, crystal-filled chambers and strong iridescence cost more. A fossil with filler or paint is worth less than an honest one, so ask what was done to it.
Ammonite for sale
Ammonite for sale on geoslut arrives in drops: each piece is one of one, photographed in daylight and labeled with its locality, and the list hears about every drop first.
Ammonite FAQ
Are ammonites extinct?
Ammonites are extinct: they died out 66 million years ago with the non-bird dinosaurs. Their closest living relatives are probably squid, octopus and cuttlefish rather than the nautilus.
How old are ammonite fossils?
Ammonite fossils are about 400 to 66 million years old, depending on the rock layer they came from. Species changed fast and spread across whole oceans, so geologists use them as index fossils to date rocks.
What is the difference between an ammonite and a nautilus?
The difference between an ammonite and a nautilus starts with time: the nautilus is alive and the ammonite is extinct. Nautilus chamber walls meet the shell in simple curves, while ammonite sutures are frilled.
Can ammonite go in water?
Ammonite can go in water for a quick rinse, but do not soak it: the shell is soft, and pyritized pieces can rust.
Sources
- Fossil ammonites, Smithsonian National Museum of Natural History
- What is an ammonite, Natural History Museum (London)
- Ammonoidea, Wikipedia