Scoria
Scoria is a dark, highly vesicular volcanic rock of basaltic to andesitic composition, riddled with gas bubbles trapped as the lava cooled and depressurized rapidly. It is denser and darker than pumice, containing thicker vesicle walls and less silica, which is why scoria typically sinks in water while pumice floats. Its red color, when present, comes from oxidized iron in the rock.
Scoria properties at a glance
| Type | Igneous Rocks |
|---|---|
| Mohs hardness | 5–6 |
| Luster | Dull, glassy |
| Fracture | Irregular, vesicular |
| Specific gravity | 1.5–2.6 |
| Transparency | Opaque |
| Common colors | Dark red, Black, Brown, Reddish brown |
| Rarity | common |
Geology & background
Scoria forms during relatively fluid, gas-rich basaltic eruptions, building up cinder cones as frothy lava fragments, called cinders, are thrown into the air and cool quickly around a vent. It also caps many basaltic lava flows as a rubbly, clinker-like crust that forms when the flow's outer surface degasses and hardens while the interior is still moving. Cinder cone fields such as Arizona's San Francisco Volcanic Field and Hawaii's Kilauea area are classic places to find fresh scoria. Rockhounds usually collect scoria for its striking red or black color and rough, popcorn-like texture rather than for rarity, since it is abundant wherever basaltic volcanism has occurred. Crushed scoria is widely used as a lightweight landscaping material and low-cost aggregate, a practical use that sets it apart from most other collectible volcanic rocks.
How to identify Scoria
Scoria is dark reddish-brown to black, riddled with rounded to irregular gas-bubble cavities called vesicles, giving it a rough, popcorn-like texture. It feels notably lighter than a solid rock of the same size but is dense enough to sink in water, unlike pumice, which floats. Hardness reflects its basaltic mineral content, roughly 5 to 6, though the vesicular structure makes it crumble more easily than solid rock. It is most often confused with pumice, which is lighter-colored, more silica-rich, and buoyant.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Scoria is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Scoria worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Construction or aggregate when suitable
- Landscape interpretation
- Education
Interesting facts
- Unlike pumice, which floats, scoria is dense enough to sink in water despite being full of gas-bubble cavities.
- Cinder cones, the classic conical volcanic hills seen across the American Southwest, are built almost entirely from accumulated scoria fragments.
- Scoria's red color comes from iron in the rock oxidizing, essentially rusting, while the lava fragments were still hot.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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