Sandstone Identification Guide

What Sandstone Looks Like

Sandstone is a clastic sedimentary rock made of cemented sand-sized grains (0.0625–2 mm), most often quartz, sometimes feldspar and rock fragments. Colors range widely — buff, tan, yellow, red, brown, gray, and white — depending on the cementing minerals and iron content (red from hematite, yellow-brown from limonite). It typically looks granular and slightly rough, and you can often see and feel individual sand grains. Bedding layers, cross-bedding, and ripple marks are common, and many sandstones are porous enough to absorb a drop of water.

Step-by-Step Field Checklist

1. Examine grain size. Use a hand lens — grains should be sand-sized (visible but pinhead-scale). If you cannot see grains it may be siltstone or shale; if pebbles are present it is conglomerate.
2. Feel the surface. Sandstone has a gritty, sandpaper-like texture; rub it and grains may rub off.
3. Look for sorting and rounding. Note whether grains are well-rounded and uniform (mature sandstone) or angular and mixed (greywacke).
4. Check for bedding. Layering, cross-beds, or ripple marks confirm a sedimentary origin.
5. Test porosity. A drop of water often soaks in and may fizz slightly with air bubbles.
6. Scratch test. Most quartz sandstone scratches glass, but the rock itself may crumble because the cement is weaker than the grains.

Key Diagnostic Tests

• Hardness: grains are quartz (Mohs 7), but rock strength depends on cement; it may scratch glass yet crumble under a hammer.
• Acid: drips of dilute HCl fizz only if the cement is calcite (calcareous sandstone).
• Cleavage/fracture: none; breaks around grains, giving a rough surface.
• Porosity: often high; absorbs water.
• Streak/density: not diagnostic; SG ~2.0–2.6 depending on porosity.

Common Look-Alikes and How to Tell Them Apart

• Quartzite: metamorphosed sandstone; it breaks through the grains rather than around them, giving a glassy, smooth fracture and a much harder, denser rock with no loose grains.
• Siltstone/mudstone: finer grains that you cannot see individually; feels smooth, not gritty.
• Conglomerate/breccia: contains rounded (conglomerate) or angular (breccia) pebbles larger than 2 mm set in a finer matrix.
• Arkose: a feldspar-rich sandstone (often pink); look for pink, blocky feldspar grains among the quartz.
• Limestone: may look pale and bedded, but limestone fizzes vigorously in acid and lacks visible sand grains.

Where Sandstone Is Found

Sandstone is one of the most abundant sedimentary rocks, forming wherever sand accumulates — deserts (wind-blown dunes), river channels, deltas, beaches, and shallow seas — then becomes buried and cemented. It dominates landscapes such as the Colorado Plateau (Zion, Arches), the Appalachian basins, and countless quarries worldwide where it is mined for building stone.