Sedimentary Rocks · common
Graywacke
Graywacke is a hard, dark, poorly sorted sandstone with a muddy matrix and mixed mineral or rock fragments.
Also known as: Greywacke, Graywacke sandstone, Greywacke sandstone, Dark sandstone.
Graywacke properties at a glance
| Type | Sedimentary Rocks |
|---|---|
| Chemical formula | Variable sandstone with quartz, feldspar, rock fragments, and clay matrix |
| Mohs hardness | 6 |
| Luster | dull |
| Streak | Gray to dark gray |
| Cleavage | Variable by mineral grains |
| Fracture | Granular to uneven |
| Specific gravity | 2.55 |
| Transparency | opaque |
| Common colors | Dark gray, Greenish gray, Brown, Black |
| Rarity | common |
Geology & background
Forms from poorly sorted sand, rock fragments, and muddy matrix, commonly deposited by underwater sediment gravity flows.
Graywacke is important in interpreting ancient submarine fans, mountain belts, and rapidly eroded source terrains.
How to identify Graywacke
- Look for dark gray sandstone with mixed grain sizes and a muddy-looking matrix.
- Graywacke is usually less clean and less pale than quartz sandstone.
- It may show bedding or graded layers in field specimens.
- The rock can be quite tough compared with softer mudrocks.
Full Graywacke identification guide: checklist, home tests and look-alikes →
Graywacke vs similar stones
Not sure it isn't something else? Side-by-side comparisons with the stones most often confused with Graywacke.
Where Graywacke is found
Graywacke occurs in many sedimentary settings worldwide; exact locality should be verified from field notes, maps, or specimen provenance.
How much is Graywacke worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Graywacke is usually valued for teaching, field context, locality, texture, fossils, color, or unusual structures rather than high gem value.
Common uses
- Field identification
- Education
- Aggregate
- Building stone when durable
Interesting facts
- Graywacke is often associated with deep marine turbidite deposits.
- It tends to look darker and dirtier than clean quartz sandstone.
- The British spelling is greywacke.
Care & safety
Graywacke can be soft, porous, crumbly, or weakly cemented depending on grain size, cement, and weathering.
Avoid inhaling dust. Do not cut, grind, or polish unknown sedimentary rocks without eye protection and a properly rated respirator.
Graywacke FAQ
What is Graywacke?
Graywacke is a hard, dark, poorly sorted sandstone with a muddy matrix and mixed mineral or rock fragments.
How much is Graywacke worth?
Entry-level Graywacke specimens sell for around $1, a typical collector specimen for about $10, and premium pieces for $75 or more. Graywacke is usually valued for teaching, field context, locality, texture, fossils, color, or unusual structures rather than high gem value.
How hard is Graywacke?
Graywacke has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is Graywacke used for?
Graywacke is used for field identification, education, aggregate, and building stone when durable.
