An agatized clam is a fossilized bivalve shell whose original material has been replaced by microcrystalline silica, preserving the structure while turning the fossil into gemstone material. Collectors prize it for its intricate internal banding and varied coloration. It occurs in sedimentary deposits where silica-rich groundwater has percolated through ancient shell beds.
Is this agatized clam?
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 agatized clam with a known reference. Agatized Clam 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. Agatized Clam leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Agatized Clam typically shows a waxy luster.
- 4Match the color rangeCompare against the expected color range: white, gray, brown, tan, red, yellow.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive.
Often confused with
Agatized Clam vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside agatized clam
Minerals reported to co-occur with agatized clam. 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
- Waxy
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector, Lapidary, Decorative
- Host rock
- Sedimentary Layers
Where rockhounds find agatized clam
3 mapped spotsClassic worldwide localities
- Florida, USA
- California, USA
- Madagascar
- Oregon, USA
Field-hunting tip
Look in sedimentary layers country, the host setting where agatized clam typically forms. If you start seeing quartz, calcite, chalcedony in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
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Find agatized clam on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
