Sandstone vs Limestone: How to Tell the Difference

You've picked up two pale rocks that look almost identical. Both are layered, both are sedimentary, both are common as dirt. But one is sandstone and the other is limestone, and telling them apart matters if you're trying to identify what you found.

Here's the thing: sandstone and limestone look similar, but they form in totally different ways and have completely different properties. One is made of sand grains, the other of ancient sea life. One is tough and gritty, the other soft and fizzy.

Let's break down how to tell them apart, what each rock is made of, and where you'll find them in the wild.

What Is Sandstone?

Sandstone is a clastic sedimentary rock made of sand-sized grains (0.06mm to 2mm) cemented together. Most sandstone is made of quartz grains because quartz is hard, durable, and resistant to weathering. The grains come from the breakdown of older rocks like granite.

Over millions of years, these grains pile up in river deltas, beaches, deserts, or shallow seas. They get buried, compacted, and cemented together by minerals like silica, calcite, or iron oxide. The result? A rock that looks and feels like solidified sand, because that's exactly what it is.

Sandstone can be tan, red, brown, yellow, gray, or white. The color depends on the cement: iron oxide makes it red or orange, silica makes it white or gray, and calcite can make it tan or cream-colored.

What Is Limestone?

Limestone is a chemical or biochemical sedimentary rock made primarily of calcium carbonate (CaCO₃). Unlike sandstone, which forms from broken rock fragments, limestone forms from the accumulation of shells, coral skeletons, and microscopic marine organisms.

When these organisms die, their calcium carbonate shells and skeletons settle to the ocean floor, pile up in massive layers, and eventually compact into solid rock. Some limestone also forms chemically when calcium carbonate precipitates directly from seawater.

Limestone is typically gray, white, cream, or tan. It often contains visible fossils: shells, coral, crinoid stems, or brachiopods. Some limestone is so packed with fossils it's called coquina or fossiliferous limestone.

How to Tell Sandstone from Limestone: The Acid Test

This is the easiest, fastest, most definitive test. All you need is a bottle of white vinegar.

Put a drop of vinegar on your rock and watch what happens:

• Limestone: Fizzes vigorously. You'll see bubbles forming immediately. This is CO₂ gas being released as the acid dissolves the calcium carbonate.
• Sandstone: No fizz, or very weak fizzing. Pure quartz sandstone won't react at all. Sandstone cemented with calcite may fizz slightly, but nothing like limestone.

The acid test works because calcium carbonate reacts with weak acids, but silica (the main component of sandstone) doesn't. This is the gold standard test for limestone.

Other Ways to Tell Them Apart

1. Texture: Gritty vs. Smooth

Sandstone feels gritty and sandy to the touch. Run your fingers over it and you'll feel individual grains. It's like touching sandpaper. Some sandstone is so poorly cemented that grains rub off on your hands.

Limestone feels smooth or chalky. It's finer-grained, often almost powdery on weathered surfaces. You won't feel individual sand grains because there aren't any. Well-polished limestone can be glassy smooth.

2. Hardness

Sandstone is harder because it's made of quartz grains. Quartz has a hardness of 7 on the Mohs scale, so well-cemented sandstone is tough. A steel knife won't scratch it.

Limestone is softer, around 3-4. A steel knife blade easily scratches limestone. You can even scratch it with a copper penny sometimes. This makes limestone easier to carve, which is why it's used for sculptures and building facades.

3. Fossils

Limestone is loaded with fossils. Look for shell imprints, coral fragments, or circular crinoid stem discs. If your rock has obvious marine fossils, it's almost certainly limestone (or a related carbonate rock like dolostone).

Sandstone rarely contains fossils. It formed in higher-energy environments (beaches, rivers, deserts) where shells and bones get broken apart. You might find trace fossils like worm burrows or footprints, but not intact shells.

4. Layering and Bedding

Both rocks show layering (bedding), but the style differs:

• Sandstone: Often has cross-bedding (angled layers) from ancient sand dunes or river currents. Layers can be thick or thin, sometimes with ripple marks preserved.
• Limestone: Usually has flat, horizontal bedding. Layers represent slow accumulation on a calm seafloor. Limestone can also be massive and structureless.

5. Color

Sandstone comes in warm colors: tan, brown, red, orange, yellow. The red and orange colors come from iron oxide cement (rust). Desert sandstone (like in Utah or Arizona) is famously red.

Limestone is usually cooler-toned: gray, white, cream, or pale blue-gray. Some limestone is nearly pure white (like chalk). Black limestone exists but is less common (usually colored by organic matter).

Where to Find Sandstone

Sandstone forms in environments where sand accumulates:

• Deserts: Ancient sand dune fields, like those that formed the massive sandstone formations in Utah, Arizona, and the Colorado Plateau.
• Beaches and coastlines: Where wave action deposits sand.
• River deltas: Where rivers dump sediment as they slow down.
• Shallow seas: Where sand washed off continents settles on the seafloor.

Famous sandstone formations include the red rocks of Sedona, Zion National Park, Arches National Park, and the sandstone cliffs of the American Southwest.

Where to Find Limestone

Limestone forms in warm, shallow marine environments where shell-forming organisms thrive:

• Ancient seabeds: Most limestone formed in shallow tropical seas millions of years ago. These areas are now exposed as dry land.
• Coral reefs: Modern coral reefs will eventually become limestone as they get buried and fossilized.
• Caves: Limestone dissolves in slightly acidic water, creating massive cave systems like Mammoth Cave and Carlsbad Caverns.

Limestone is common in the Midwest (Indiana, Kentucky, Missouri), the Appalachian region, Texas (especially the Hill Country), and Florida (the entire state is basically limestone).

Why the Difference Matters

Beyond identification, sandstone and limestone have different uses:

Sandstone: Used for building stone, paving, and decorative rock. It's durable but porous, so it can weather and stain. Sandstone aquifers store groundwater in their pore spaces.

Limestone: Used for cement, concrete, agricultural lime (neutralizes acidic soil), and building stone. It's softer, so it's easier to carve. Limestone also forms karst landscapes with sinkholes and caves.

Quick Comparison: Sandstone vs. Limestone

Here's the cheat sheet:

• Composition: Sandstone = quartz grains. Limestone = calcium carbonate.
• Texture: Sandstone = gritty. Limestone = smooth.
• Acid test: Sandstone = no fizz. Limestone = strong fizz.
• Hardness: Sandstone = hard (6-7). Limestone = soft (3-4).
• Fossils: Sandstone = rare. Limestone = common.
• Color: Sandstone = warm tones. Limestone = cool tones.

Ready to Identify Your Rock?

If you're still not sure whether your rock is sandstone or limestone, grab Rock Identifier and snap a photo. The AI can analyze texture, color, and structure to give you an instant ID.

And once you know what you've got, you can dive deeper into the three types of rocks or explore how to identify rocks using simple field tests.