Gleamite

Rock identification chart

By The Gleamite desk, Identification research · updated

This chart identifies rocks by four properties you can check at a kitchen table: grain size, layering, hardness against a steel nail, and reaction to vinegar. It covers the twenty rocks that account for most real finds, split into igneous, sedimentary and metamorphic tables, each row naming the check that separates lookalikes.

How to use this chart

Work the checks in order, because each one eliminates more candidates than the one after it. Grain sorts the three rock families. Layering splits the assembled rocks from the squeezed ones. Hardness and vinegar settle the pale lookalikes that photos and eyeballs cannot.

  1. Step 1Look at the grainInterlocking crystals or glass point igneous. Sand grains, pebbles or hardened mud point sedimentary. Wavy bands or aligned sparkle point metamorphic.
  2. Step 2Look for layersFlat beds that split are sedimentary. Contorted light-and-dark bands are metamorphic. No structure at all usually means igneous.
  3. Step 3Bracket the hardnessTry a fingernail, then a copper coin, then a steel nail. Note the first thing that leaves a scratch — that brackets the rock on the Mohs scale.
  4. Step 4Drop vinegar on itFizzing means carbonate: limestone, marble or a calcite vein. Silence rules out an entire family in one move.

A wet rock tells the truth more often than a dry one. Dust and weathering hide grain and banding; a dip in water restores both, which is why every beach photo guide says the same thing — and why the rock identifier app asks for a dry shot and a wet shot. One scope note: this page sorts rocks, which are mixtures. A single mineral — a crystal, an ore, a metallic lump — belongs to the mineral identification chart and its luster-hardness-streak columns instead.

Igneous rocks: frozen from melt

Igneous rocks solidified from magma or lava. The grain records the cooling speed: slow cooling underground grows visible crystals, fast cooling at the surface leaves fine grain or glass. The two-question version of this table — coarse or fine, light or dark — is worked in full in igneous rock identification.

RockLookHardness feelThe telltale
GraniteSpeckled, light, coarse crystalsScratches glassSalt-and-pepper crystals visible at arm’s length
GabbroSpeckled but dark, coarseScratches glassGranite’s dark twin, no quartz sparkle
RhyoliteLight, fine-grained, sometimes bandedScratches glassGranite’s fast-cooled twin
BasaltDark gray to black, fine, heavyNear steel nailThe most common lava rock on Earth
ObsidianBlack glass, curved shell-like breaksScratches glassNo crystals at all; edges can cut
PumicePale, frothy, full of holesCrumblesFloats on water
ScoriaDark, holey, heavier than pumiceRoughSinks where pumice floats

Sedimentary rocks: assembled in layers

Sedimentary rocks are debris — sand, mud, shells, pebbles — pressed and cemented into beds. Layering is the family signature, and the vinegar test finds the carbonates among them. The grain-size ladder and the tooth test that extends this table live in sedimentary rock identification.

RockLookHardness feelThe telltale
SandstoneVisible sand grains, grittyCoin to nailRubs off individual grains
ConglomerateRounded pebbles in cementVariesNature’s concrete, pebbles smooth
BrecciaAngular fragments in cementVariesConglomerate with sharp-edged clasts
ShaleThin gray sheets, splits flatFingernailSmells earthy when breathed on wet
LimestonePale, dull, sometimes fossilsCoinFizzes in vinegar
Chert and flintWaxy, breaks in curved shellsScratches glassSparks against steel
CoalBlack, light for its sizeFingernail to coinLeaves black marks on hands

Metamorphic rocks: squeezed and recrystallized

Metamorphic rocks are older rocks transformed by heat and pressure. The signatures are alignment — bands, sheets, oriented sparkle — and a re-crystallized density the parent rock lacked. The slate-to-gneiss grade ladder behind this table is climbed rung by rung in metamorphic rock identification.

RockLookHardness feelThe telltale
SlateDark, splits into flat platesCoin to nailRings faintly when tapped
PhylliteLike slate with a silky sheenCoin to nailWrinkled satin surface
SchistSparkly, flaky, mica-richVariesGlitters from aligned mica flakes
GneissLight and dark bandsScratches glassBanding is contorted, not flat beds
MarblePale, sugary, sometimes veinedCoinFizzes in vinegar
QuartzitePale, sugary, glassy glintScratches glassSilent in vinegar where marble fizzes

The hardness kit, calibrated

The chart’s hardness column assumes three household tools. Their Mohs positions are what make the bracketing work, so the kit is worth memorizing once.

ToolMohs valueWhat scratching means
Fingernailabout 2.5Scratches gypsum, shale, selenite, coal
Copper coinabout 3.5Scratches calcite, limestone, marble
Steel nailabout 5.5Scratches apatite and most fine-grained gray rocks
Glass bottleabout 5.5Anything that scratches it is quartz-hard or better

The order of operations matters: always try the soft tool first. A steel nail proves little on its own, because most rocks fall below it; a fingernail that leaves a mark eliminates half the chart instantly.

Start from color when that is all you have

Color is the weakest single property — iron stains everything — but it is often the first thing known, and each shade has its own short list of candidates and checks.

The stone isStart here
Pink to rosePink rock identification
Red to brickRed rock identification
Orange to rustOrange rock identification
Yellow to mustardYellow rock identification
Green in any shadeGreen rock identification

When the chart is not enough

Three finds regularly defeat property tables. Pale sugary rocks — the marble, quartzite and dense limestone triangle — need the vinegar-plus-glass sequence, not either check alone. Suspected gold and metallic glints need a streak test, because color in sunlight is exactly what pyrite fakes. And anything you suspect might be worth money deserves the elimination sequence in how to identify valuable rocks before any paid appraisal.

Regional guides beat generic ones where geology is distinctive: Great Lakes beach finds — Petoskey stones, agates, Yooperlites — have their own chart in Michigan rock identification, with the same treatment for Missouri, Ohio, Texas and Georgia. Banded translucent pebbles get the full treatment in how to identify agate. Single crystals with visible faces are a different problem again; the crystal identifier covers those checks.

Questions people ask

Is there a printable rock identification chart?
Print this page — the tables are plain HTML and print cleanly in black and white. Take the hardness kit table too: the chart is only useful alongside a fingernail, a copper coin and a steel nail, and those three cover the full range the chart asks about.
What are the four steps to identify a rock?
Check grain first: visible crystals, sand grains, or glassy smoothness. Check structure second: layers, bands, or uniform texture. Then scratch it against a fingernail, copper coin and steel nail to bracket hardness. Finally, drop vinegar on it — fizzing means a carbonate rock like limestone or marble.
How do I tell igneous, sedimentary and metamorphic rocks apart?
Igneous rocks look like frozen liquid — interlocking crystals or glass with no layers. Sedimentary rocks look assembled — grains, pebbles or mud pressed into beds that often split. Metamorphic rocks look squeezed — wavy bands, aligned sparkles or a sugary re-crystallized texture that rings when tapped.
What is the most common rock people misidentify?
Quartzite passes for marble constantly: both are pale, sugary and dense. Vinegar settles it in ten seconds — marble is calcite and fizzes, while quartzite is quartz and stays silent, then proves the point by scratching glass. Slag passing for meteorite is the runner-up.
The Gleamite deskWrites every identification chart on this site from named mineralogical properties — Mohs values, streak colors, specific gravity — sourced from USGS and standard references on a stated date, and runs the same hardness, streak and vinegar checks it tells readers to run.
Sources