Ammonite
Ammonites were shelled cephalopods (relatives of today's nautilus and octopus) that went extinct with the dinosaurs 66 million years ago. The Cretaceous Interior Seaway that once split North America left millions of ammonite fossils across the Great Plains. Some rare specimens preserve their original shell as ammolite — a gem-quality iridescent material used in fine jewelry.
Physical Properties
Chemical Formula
Fossil cephalopod (aragonite/calcite shell)
Hardness (Mohs)
3.5–4
Color
Gray, brown, iridescent (ammolite)
Luster
Dull to iridescent (gem quality)
Crystal System
N/A (fossil)
Streak
White
Cleavage
N/A
Specific Gravity
2.7
🔍 Field Identification Tips
Spiral coiled shell divided into chambers by curved partitions (septa). The suture lines where septa meet the outer shell create complex fractal patterns — a key ID feature. Found in marine sedimentary rock (limestone, chalk, shale). Not to be confused with nautiloids (simpler suture patterns).
Varieties & Forms
📍 Where to Find Ammonite in the US
Uses & Applications
Fossil collecting, jewelry (ammolite is an official gemstone), decorative specimens.
💰 Collector Value Guide
Small common ammonites: $5–$50. Large display specimens: $100–$2,000. Pyritized ammonites: $50–$500. Gem ammolite: $50–$1,000/ct.
Find Ammonite With Our Tools
Use the We Lick Rocks app to identify specimens, find dig sites, and connect with local rockhounds.