A clam geode is a fossilized mollusk shell that has been hollowed out and filled with secondary crystal growth, typically microcrystalline silica like chalcedony or quartz. These specimens are highly prized for their unique combination of preserved biological structure on the exterior and sparkling crystalline interiors. They are commonly sourced from ancient seabed deposits where silicification occurred during the fossilization process.
Is this clam geode?
5-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 clam geode with a known reference. Clam Geode sits at Mohs 6.5-7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Clam Geode leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Clam Geode typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, gray, brown, blue, translucent.
- 5Look at form & habitCrystal system: trigonal. Typical habit: botryoidal.
Often confused with
Clam Geode vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside clam geode
Minerals reported to co-occur with clam geode. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SiO₂
- Mohs hardness
- 6.5-7
- Density
- 2.6-2.7 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Botryoidal
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector, Lapidary, Decorative
- Host rock
- Sedimentary
- Typical price
- $10-150 depending on size and crystal quality
Where rockhounds find clam geode
1 mapped spotsClassic worldwide localities
- Florida, USA
- Texas, USA
- Australia
- Morocco
Field-hunting tip
Look in sedimentary country — that is the host setting where clam geode typically forms. If you start seeing quartz, chalcedony, calcite in float, you are in the right ground. Field specimens usually show a botryoidal habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Virginia — start trip planning there.
Common questions
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Find clam geode on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
