Greywacke is a texturally immature, poorly sorted sandstone characterized by its dark color and high proportion of lithic fragments and rock grains. Collectors often find it in mountainous regions where deep-water marine sequences have been tectonically uplifted and exposed. It is easily identified by its gritty feel and lack of distinct bedding compared to typical sedimentary rocks.
Is this greywacke?
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 greywacke with a known reference. Greywacke sits at Mohs 6-7, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Poorly sorted angular grains in a compact, clay-fine matrix. Grain size: Sand- to gravel-sized grains; matrix over 15 percent.
- 3Pick out the mineralsComposition: Quartz, feldspar and small rock fragments in a clay matrix. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull.
- 5Match the color rangeCompare against the expected color range: gray, dark gray, brownish gray.
Often confused with
Greywacke vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside greywacke
Minerals reported to co-occur with greywacke. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 6-7
- Density
- 2.6-2.7 g/cm³
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Construction aggregate, Decorative stone, Educational
- Host rock
- marine sedimentary basins
- Texture
- Poorly sorted angular grains in a compact, clay-fine matrix
- Grain size
- Sand- to gravel-sized grains; matrix over 15 percent
- Composition
- Quartz, feldspar and small rock fragments in a clay matrix
Where rockhounds find greywacke
Classic worldwide localities
- New Zealand
- Scotland
- Wales
- California, USA
Field-hunting tip
Where it forms: marine sedimentary basins. If you start seeing quartz, feldspar, mica in float, you are in the right ground.
More greywacke photos
Common questions
How do you identify greywacke?+
Where is greywacke found?+
What rocks look like greywacke?+
What minerals are found with greywacke?+
What kind of rock does greywacke form in?+
What is greywacke used for?+
Find greywacke on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
