Black chert is a microcrystalline variety of silica that typically occurs as nodules within limestone or chalk formations. It is well-known to rockhounders for its conchoidal fracture, which creates exceptionally sharp edges and made it a primary material for prehistoric tool manufacture.
Is this black chert?
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 black chert with a known reference. Black Chert 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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: waxy.
- 4Match the color rangeCompare against the expected color range: black, dark gray.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive.
Often confused with
Black Chert vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside black chert
Minerals reported to co-occur with black chert. 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
- 2.6-2.65 g/cm³
- Streak
- White
- Luster
- Waxy
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Common
- Uses
- Lapidary, Decorative, Knapping
- Host rock
- sedimentary limestone or chalk deposits
Where rockhounds find black chert
1 mapped spotsClassic worldwide localities
- United Kingdom
- France
- USA
- Denmark
- Belgium
Field-hunting tip
Where it forms: sedimentary limestone or chalk deposits. If you start seeing calcite, dolomite, chalcedony in float, you are in the right ground. Typical habit: massive. 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.
More black chert photos
Common questions
How do you identify black chert?+
Where is black chert found?+
Can I find black chert in the United States?+
What rocks look like black chert?+
What minerals are found with black chert?+
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Find black chert on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
