Potomac Marble is a decorative sedimentary rock composed of rounded limestone clasts cemented together in a colorful matrix. It is famous in American history for being used in the columns of the U.S. Capitol building and is often found as a polished stone in architectural or display collections.
Is this potomac marble?
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 potomac marble with a known reference. Potomac Marble sits at Mohs 3-4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Potomac Marble leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Potomac Marble typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: red, green, brown, white.
- 5Look at form & habitTypical habit: massive.
Often confused with
Potomac Marble vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside potomac marble
Minerals reported to co-occur with potomac marble. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3-4
- Density
- 2.6-2.8 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Opaque
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Common
- Uses
- Ornamental, Decorative, Lapidary
- Host rock
- Sedimentary
- Typical price
- $10-100 per specimen
Where rockhounds find potomac marble
1 mapped spotsClassic worldwide localities
- Maryland, USA
- Potomac River Valley, USA
Field-hunting tip
Look in sedimentary country — that is the host setting where potomac marble typically forms. If you start seeing calcite, dolomite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Pennsylvania — start trip planning there.
Common questions
How do you identify potomac marble?+
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Find potomac marble on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
