Orthoceras is an extinct genus of nautiloid cephalopod characterized by its distinctive straight, conical shell. These fossils are frequently found embedded in limestone matrices and are popular in lapidary for their aesthetic internal chamber patterns, which often show white calcite crystallization.
Hardness
3-4
Mohs
Luster
Dull to vitreous
Transparency
Opaque
Is this orthoceras?
4-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch orthoceras with a known reference. Orthoceras sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Name the organismDecide what left the fossil before worrying about the rock. Organism: Straight-shelled nautiloid (genus Orthoceras). Age: Middle Ordovician (strict sense); the name was once used for many Paleozoic and Triassic straight-shelled nautiloids.
- 3Read the lusterHold the specimen under a strong light. Luster: dull to vitreous.
- 4Match the color rangeCompare against the expected color range: black, brown, gray, white.
Often found alongside orthoceras
Minerals reported to co-occur with orthoceras. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3-4
- Density
- 2.6-2.9 g/cm³
- Luster
- Dull to vitreous
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Collector, Decorative, Lapidary
- Host rock
- limestone
- Organism
- Straight-shelled nautiloid (genus Orthoceras)
- Age
- Middle Ordovician (strict sense); the name was once used for many Paleozoic and Triassic straight-shelled nautiloids
Where rockhounds find orthoceras
Classic worldwide localities
- Morocco
- Sweden
- Estonia
- United States
Field-hunting tip
Where it forms: limestone. If you start seeing calcite, limestone in float, you are in the right ground.
More orthoceras photos
Common questions
How do you identify orthoceras?+
Mohs hardness is 3-4. Organism: Straight-shelled nautiloid (genus Orthoceras). Luster: dull to vitreous. Common colors include black, brown, gray, white.
Where is orthoceras found?+
Notable localities include Morocco; Sweden; Estonia; United States.
What minerals are found with orthoceras?+
Orthoceras commonly co-occurs with Calcite, Limestone. Spotting any of these in float or country rock is a useful trip signal.
What kind of rock does orthoceras form in?+
Where it forms: limestone. Working float back to the host body is the standard way to chase a fresh occurrence.
What is orthoceras used for?+
Orthoceras is used in collector, decorative, lapidary.
Find orthoceras on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
