Rock Identification Chart: Igneous, Sedimentary, Metamorphic

A rock identification chart sorts a rock by what you can see before it asks what the rock is called. The Regents Earth Science flow chart posted by the North Inlet–Winyah Bay reserve makes the first cut on texture, and the LibreTexts charts by Page Quinton then read each rock group by texture and composition.

• Crystals you can see, packed together with no space between them: igneous or metamorphic.
• Broken pieces (clasts) of other rocks or minerals, cemented together: sedimentary.
• No crystals and no clasts, a glassy or very fine, uniform material: volcanic glass, chert, or a fine sedimentary rock.

Then go to the section below for that group. Work on a fresh, dry, broken surface with a hand lens and a steel nail within reach.

Igneous rocks are read on two axes: colour, which tracks composition, and crystal size, which tracks how fast the melt cooled. The Regents lab gives these bands.

Colour, by the share of dark minerals:

• Light (felsic): about 10% dark, 90% light.
• Intermediate: roughly half and half.
• Dark (mafic): about 90% dark.
• Green (ultramafic): dark with green, from the mantle.

Crystal size:

• Glassy: no visible crystals.
• Fine: crystals under 1 mm, formed at the surface (extrusive, volcanic).
• Coarse: crystals 1–10 mm, formed underground (intrusive, plutonic).
• Very coarse: crystals over 10 mm.
• Vesicular: full of gas holes.

Where they meet, using the MSA Rock Key's descriptions:

• Light and coarse: granite, white, grey or pink feldspar with smoky quartz and black biotite specks, crystals randomly arranged.
• Light and fine: rhyolite, light grey, tan or reddish, often with scattered larger crystals and flow bands.
• Intermediate and coarse: diorite, like a dark granite with little or no mica.
• Dark and coarse: gabbro, dark green to black, grains larger than rice.
• Dark and medium: diabase, grains visible but smaller than rice.
• Dark and fine: basalt, dark grey to black, crystals often microscopic, sometimes with gas bubbles.
• Glassy: obsidian, black glass that breaks with a curved, shell-like fracture.
• Vesicular and light: pumice, so full of bubbles that most pieces float.
• Vesicular and dark: scoria, fewer, larger bubbles and moderately heavy.

The Regents sedimentary lab splits these rocks three ways, then names clastic rocks by grain size and the others by what they are made of.

Clastic (bits of other rock), by grain size:

• Gravel with rounded pebbles: conglomerate. The MSA key says the pebbles are the important observation.
• Gravel with jagged, blocky pieces: breccia.
• Sand: sandstone. Usually rounded grains of one size, red, brown, grey, white or yellow; a steel nail scrapes sand grains off it.
• Clay: shale. Particles too fine to see; the MSA key notes it usually smells like wet mud when moistened.

Crystalline (precipitated from water): named by composition, such as rock salt, rock gypsum and some limestones.

Bioclastic (pieces of living things): fossil-rich limestone, and coal where you can see plant material, which the Regents lab ties to swamps.

The quick test for limestone: the MSA key says the powdered rock usually fizzes in white vinegar, and unlike marble it has no visible crystals.

Metamorphic rocks are igneous or sedimentary rocks changed by heat and pressure. The Regents lab reads them by texture first: foliated, where minerals are aligned or banded, or non-foliated, a granular texture that is a blurred version of the parent rock.

Foliated, from fine to coarse:

• Slate: very fine grained, thin smooth flat layers, splits into thin sheets and often scratches glass with a little difficulty. Shale does not split this cleanly.
• Schist: silvery to dark mica or green chlorite in thin, often wavy layers; splits easily along the mica.
• Gneiss: ribbons or stripes of minerals, fairly coarse, breaks into blocks rather than along the layers. Its crystals are lined up, unlike granite's.

Non-foliated:

• Marble: often white, visible crystals, will not scratch glass; the powder often fizzes in vinegar. If it does not, it may be dolomitic marble.
• Quartzite: white to reddish, easily scratches glass, and breaks through the sand grains rather than around them as sandstone does.
• Serpentinite: green to greyish-green and slippery to the touch.

These are the pairs the MSA Rock Key singles out with a direct test, which is what a photo cannot do.

• Granite or gneiss? Random crystals mean granite; crystals lined up in bands mean gneiss.
• Marble or quartzite? Drag it across glass lying flat on a table. Quartzite scratches glass easily; marble does not.
• Sandstone or quartzite? Look at a broken edge with a lens: sandstone breaks around its grains, quartzite through them.
• Pumice or scoria? Pumice is light grey and most pieces float; scoria is darker, heavier and has fewer, larger holes.
• Shale or slate? Slate splits into thin, flat sheets and may scratch glass; shale is softer and smells of wet mud when damp.

For the individual rocks, our pages on granite, basalt, marble and quartzite list the home tests.

Weathered surfaces, rocks cut and polished for landscaping, and small pebbles with no fresh face all resist charts. Break or chip a corner to expose a fresh surface, following the safety rules on the step-by-step method page. Then photograph that surface; RockLens will name the likely rock, and the chart above tells you which one test to run to confirm it. For a hands-on answer, the USGS recommends your state geological survey or a local gem and mineral club.

Research and drafting used AI assistance, and every statement was checked against the sources listed below. The charts cited are teaching materials: the Regents Earth Science labs posted by the North Inlet–Winyah Bay National Estuarine Research Reserve, the MSA Rock Key by Don Peck, and the rock identification charts by Page Quinton on LibreTexts (CC BY-SA 4.0).