Identification guide · Sedimentary Rocks
How to Identify Conglomerate
Conglomerate is identified by its coarse, rounded to well-rounded clasts, ranging from pebble to boulder size, set in a finer sandy or muddy matrix, with overall color and luster varying with the composition of both clasts and matrix. Hardness has no single value since it depends on the individual clasts and cement, though the rock generally resists scratching if well cemented. Fracture typically runs through the matrix or around clasts rather than through them. It is distinguished from breccia mainly by rounded rather than angular fragments.
Conglomerate identification checklist
| Color | Variegated, Gray, Brown, Red, Tan | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Variable; dull matrix to vitreous clasts | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 3–7 | A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. |
| Fracture | Uneven; breaks around or through clasts | |
| Specific gravity | 2.4–2.7 | Ordinary rock-like heft (specific gravity about 2.55), similar to quartz. |
Conglomerate look-alikes and how to tell them apart
Metaconglomerate is harder (Mohs 6.5) than Conglomerate (Mohs 3–7), so Metaconglomerate will scratch Conglomerate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Conglomerate vs Metaconglomerate →
Banded Iron Formation is harder (Mohs 5–7) than Conglomerate (Mohs 3–7), so Banded Iron Formation will scratch Conglomerate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Conglomerate vs Banded Iron Formation →
Conglomerate is harder (Mohs 3–7) than Dolomite (Mohs 3.5–4), so Conglomerate will scratch Dolomite and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Conglomerate vs Dolomite →
Rhyolite is harder (Mohs 5.5–7) than Conglomerate (Mohs 3–7), so Rhyolite will scratch Conglomerate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
Conglomerate identification FAQ
How can you tell if a rock is Conglomerate?
Check the color (variegated, gray, brown, and red), the variable; dull matrix to vitreous clasts luster, a Mohs hardness of 3–7. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Conglomerate?
Conglomerate has a Mohs hardness of 3–7. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
What is the difference between Conglomerate and Metaconglomerate?
Conglomerate (Mohs 3–7, variable; dull matrix to vitreous clasts luster) and Metaconglomerate (Mohs 6.5, dull luster) look alike, but hardness, streak and heft usually separate them. See the full Conglomerate vs Metaconglomerate comparison for a side-by-side table.
Is Conglomerate worth anything?
Entry-level Conglomerate specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Conglomerate?
Conglomerate is most often variegated, gray, brown, and red. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
