In-Depth Guide

Mohs Hardness Scale Explained (With Everyday Objects)

The scratch test is the single most useful test in rock identification. Here is what the scale measures, how to test with things in your pocket, and how to read the results.

Ask a geologist to name one tool and most will say a fingernail. That is not a joke — it is the Mohs scale in action. Hardness, meaning resistance to being scratched, splits the rock world into useful groups faster than almost any other property. A rock your fingernail can scratch belongs to a short list. A rock that scratches window glass belongs to a different short list. This guide explains the scale, shows you how to test with everyday objects, and — just as important — explains what the numbers do not mean.

Where the scale came from

In 1812 the German mineralogist Friedrich Mohs ranked ten common minerals by whether each could scratch the ones below it. Talc, the softest, got 1. Diamond, which scratches everything and is scratched by nothing else, got 10. The United States Geological Survey and university extension services still teach Mohs' ranking today because it needs no equipment: the reference minerals are common, and between them sit a handful of household objects whose hardness is well known.

The full scale, with the minerals Mohs chose:

The pocket test kit: everyday objects

You rarely carry reference minerals, but you almost always carry the four testers that matter for the rocks on this site:

• Fingernail — about 2.5. If it scratches the rock, the rock is softer than 2.5: think shale, gypsum, or a weathered, crumbly surface. Your nail is the first gate.
• Copper penny — about 3. Use a solid copper feel and firm pressure. If the penny scratches it but your nail does not, the rock sits near 3 — the signature hardness of calcite, and therefore of limestone and marble.
• Steel nail or knife blade — about 5.5. If steel scratches the rock, it is below 5.5 (apatite, fluorite, most "medium" finds). If the rock instead leaves the steel looking polished and takes no groove, you are above 5.5.
• Glass plate or bottle — about 5.5 to 6. The classic field test. A rock that scratches glass is at least about 6, which puts quartz (7), flint (7), granite and gneiss in play — and rules out the carbonates and most layered rocks.

A porcelain streak plate (about 7) and a steel file (about 6.5) round out a home kit for a few dollars, but the four objects above resolve the great majority of beginner finds.

How to run a scratch test properly

Technique matters more than force. First, pick a fresh, inconspicuous spot — ideally a broken edge rather than a weathered face, because weathered rinds are often softer than the rock beneath. Press the edge of your tester firmly and draw it across the surface once, over a short distance. Then wipe the mark with your thumb and look again. A true scratch is a groove that survives wiping and catches a fingernail dragged across it. A pale line that wipes away is powdered metal or mineral dust from the tester, not a scratch in the rock — the most common beginner error, and the reason soft steel can seem to "scratch" hard quartz. Work from softest to hardest tester, stop as soon as one bites, and test in two spots if the first result surprises you. Test on a part of the specimen you do not mind marking: a scratch is permanent, so keep prized faces clean and use a hidden edge.

Reading your result

Translate the outcome into a range, not a single number. "Penny scratches it, nail also scratches it, but it seemed harder than the penny test suggested" means roughly 3 to 4. "Nothing in my pocket scratches it, and it scratches glass" means 6 or above, which combined with a glassy look is nearly always quartz or flint. Ranges are honest: a rock is a mixture of minerals, so granite — quartz (7) mixed with feldspar (6) and mica (2.5) — tests around 6 to 7 depending on which grain your nail lands on. That is why the identifier asks for hardness as soft, medium or hard rather than a precise figure.

What the scale does not tell you

The Mohs scale is a ranking, not a ruler. The gap between 9 and 10 is far larger in absolute terms than the gap between 1 and 2; diamond is roughly four times harder than corundum, while talc and gypsum are close cousins. Hardness is also not toughness. Quartz is hard but brittle; jade is only about 6.5 to 7 yet famously tough because of its interlocking fibres; and a hard rock can still split cleanly along cleavage planes, as slate does. Finally, hardness says nothing about value — most pretty finds are quartz-family stones whose worth is in the finding, not the selling. Use the scale for what it is: the fastest sorting tool you own. Pair it with streak, lustre and the six-step method and you have a complete beginner's laboratory, most of it already in your pocket.

In-Depth Guide

How to Identify Rocks: A Beginner's Step-by-Step Chart

The complete method behind this site: the pocket kit, the six tests in order, a worked example, and the errors that catch every beginner once.

Every rock identification — whether it happens in a university lab or on a Lake Superior beach — follows the same logic: observe, test, eliminate. You describe what the rock looks like, run a few simple physical tests, and cross off everything your observations rule out. What survives is usually a single name, or at worst a pair you can separate with one more test. This guide walks the full process slowly, the way the interactive identifier runs it quickly.

The pocket kit

You need less than you think. Your own fingernail (hardness about 2.5) is tester number one. Add a copper penny (about 3), a steel nail or pocketknife blade (about 5.5), and a small piece of glass or a glass bottle (about 5.5–6) and you can place any find into a hardness range. An unglazed porcelain tile — the back of a bathroom tile works — gives you streak. A cheap 10× hand lens reveals grain too small for the naked eye. Household white vinegar stands in for the geologist's dilute hydrochloric acid in the carbonate test; it fizzes more weakly, so give it a minute and watch closely. That is the whole laboratory: it fits in a jacket pocket and costs almost nothing.

Steps 1–2: Colour and texture

Start by washing the rock. Mud and algae lie; a wet rock also shows truer colour, though you should record the colour dry, in daylight, since that is how the charts are written. Colour is a filter, never a verdict. Black might be basalt, obsidian, coal, shale, flint or tourmaline; pink might be rose quartz, feldspar, marble or rhodonite. Note the colour, then move on quickly.

Texture is more honest, because texture records how the rock formed. Ask three questions. Can I see individual grains or crystals? Coarse interlocking crystals mean slow cooling underground — granite, gneiss. Is it gritty? Sand-sized grains you can feel, and sometimes rub loose, mean sandstone or conglomerate. Is it layered? Bands and sheets point to sedimentary shale or metamorphic slate and schist. A frothy, hole-punched lightness is pumice; a hollow centre lined with crystals is a geode. By the end of step two you usually know the rock's family — igneous, sedimentary or metamorphic — and families eliminate two-thirds of the chart at a stroke.

Steps 3–4: Hardness and streak

Hardness is the great separator, covered in depth in our Mohs scale guide. Work upward: fingernail, penny, nail, then try scratching glass. Remember the wipe test — rub the mark with your thumb; real scratches stay, metal dust wipes away. Record a range ("harder than the penny, softer than the nail: about 3–5") rather than a false precision. One warning: rocks are mixtures. Granite contains quartz at 7 and mica at 2.5, so a scratch may bite into one grain and skate off another. Test a couple of grains and take the general answer.

Streak is step four: drag the rock once, firmly, across the unglazed porcelain. Most rocks in this chart — granite, quartz, limestone, marble, sandstone — leave a white or barely visible streak, which is itself useful information. The dramatic exceptions make streak famous: rust-red betrays hematite no matter how silvery the specimen looks, and a black, sooty streak combined with suspicious lightness points straight to coal or graphite. Streak plates are about hardness 7, so quartz-family rocks will not powder on them — no streak at all is also a result, and it means "harder than the plate."

Steps 5–6: Lustre, cleavage and the acid test

Lustre describes how a surface handles light: glassy like obsidian and quartz; waxy like flint; pearly or glittering where mica flakes catch the sun, as in schist; metallic in pyrite; dull and earthy in shale. Lustre is the tiebreaker among same-coloured rocks — it is what separates glassy obsidian from dull coal at a glance.

Finally, structure. Cleavage is a tendency to split along flat planes: slate and shale split into sheets, mica peels into flakes, calcite breaks into little rhombs. The opposite habit — curved, shell-like conchoidal fracture — is the mark of glassy materials: obsidian, flint and quartz. The acid test closes the loop for pale, medium-soft rocks: a drop of vinegar on a scratched patch of surface fizzes on limestone and, more gently because it is recrystallised, on marble. Both are hardness 3, so the penny test plus the fizz is conclusive.

A worked example

Suppose you pick up a dark rock on a Pacific Northwest beach. Colour: black to dark grey. Texture: fine, no visible grains, a few pinhead holes — likely igneous, fast-cooled. Hardness: your nail does nothing, the penny does nothing, the steel nail barely marks it, and it just scratches glass — around 6. Streak: pale grey-white. Lustre: dull to slightly satiny, not glassy. It is dense and heavy in the hand. Glassy rules in obsidian, but the lustre is wrong and obsidian at 5.5 would not scratch glass so willingly; the tiny holes are vesicles from escaping gas. The answer is basalt — confirmed in the rock chart, and contrasted with its three black look-alikes in the black rocks guide.

Five beginner mistakes

1. Identifying by colour alone. Colour is the most variable property a rock has; always confirm with hardness.
2. Testing the weathered skin. Rinds are softer and paler than the fresh rock. Test a chipped edge.
3. Mistaking metal dust for a scratch. Wipe every mark before you believe it.
4. Skipping the location. A find's geology is local. Note where it came from; it eliminates impossibilities.
5. Expecting one test to do everything. Identification is elimination. Three agreeing tests beat one dramatic one.

With the method in hand, put it to work: run your next find through the step-by-step identifier, and keep the rock chart open beside you. Ten finds from now, you will be running the six steps without thinking — which is exactly how geologists do it too.

In-Depth Guide · Colour Guides

Black Rocks: How to Tell Basalt, Obsidian, Coal and Shale Apart

Four black rocks account for most "what is this black stone?" questions. They differ in weight, shine, hardness and the way they break — and each needs a different test to prove it.

Black is the most common colour of mystery rock, and the least helpful on its own. Basalt, obsidian, coal and shale are all black, all common across the United States, and all constantly mistaken for one another — yet they could hardly be more different. One is frozen lava, one is natural glass, one is fossilised plants, and one is hardened mud. The good news: you can separate all four in about a minute with your hands, a penny, a steel nail and — if you want the final proof on shale — a drop of water. Here is each rock, then the head-to-head tests.

Basalt: the plain, heavy one

Basalt is lava that cooled quickly at the surface, so its crystals never had time to grow: the texture is fine and uniform, with grains too small to see without a lens. Fresh basalt is dark grey to black, dense and noticeably heavy for its size, with a dull to faintly satiny lustre. Look closely for vesicles — tiny round holes left by gas bubbles — which are basalt's quiet signature; flood-basalt flows of the Pacific Northwest and the Lake Superior shoreline are full of them. Basalt is hard, around 6 on the Mohs scale: a steel nail struggles, and it will scratch glass. Its streak is pale grey to white. If your black rock is heavy, plain, fine-grained and hard, basalt is the answer nine times out of ten.

Obsidian: the glass

Obsidian is lava that cooled so fast no crystals formed at all — it is a natural glass, and it behaves like one. The lustre is unmistakably glassy, reflecting light in smooth, bright sheets. Edges are sharp, and broken surfaces curve in the shell-like pattern called conchoidal fracture, the same pattern you see on a chipped bottle. Obsidian is about 5.5 in hardness: it will scratch glass weakly to moderately, and a steel file can just mark it. It is moderately heavy — lighter than basalt of the same size, heavier than coal. Most obsidian is jet black, though mahogany-brown and snowflake-flecked varieties exist. The test is your eyes and fingertips: if it looks and feels like thick black bottle glass, it is obsidian, and nothing else on this list comes close.

Coal: the lightweight

Coal is compressed ancient plant matter, and the giveaway is weight — or the lack of it. A fist-sized lump of coal feels startlingly light next to any true rock, because it is mostly carbon rather than dense mineral. Its lustre ranges from dull to a satiny, almost greasy shine, often in alternating bright and dull bands. Coal is soft, around 2.5 to 3: a copper penny scratches it, and sometimes a fingernail nearly can. It may leave a black, sooty smudge or streak on your fingers and on a streak plate — none of the other three does that. Coal also breaks into blocky, brittle chunks rather than sheets or curves. If it is black, light, soft and dirties your hand, it is coal; a jet-black, polished lightweight carving stone is usually jet, coal's jewellery-grade cousin.

Shale: the splitter

Shale is mud turned to stone, and it keeps mud's layered habit: it splits readily into thin, flat, papery sheets along its bedding planes — a property geologists call fissility. Colours run grey to black (the black comes from organic matter), the lustre is dull and earthy, and the feel is smooth but never glassy. Shale is soft, about 3: a penny scratches it, and thin edges can be nicked with a fingernail. A fresh piece may smell faintly earthy or petrol-like when breathed on or wetted. It will not scratch glass, it is not especially light, and it never shows the curved glass fractures of obsidian. If your black rock wants to come apart in sheets like the pages of a book, it is shale. Its metamorphosed relative slate also splits — but slate is harder (5.5+), denser, and rings faintly when a thin piece is tapped.

Head to head: the one-minute test sequence

Run the tests in this order and stop when the answer is clear. First, heft it. Suspiciously light means coal — confirm with the penny and the sooty streak. Second, look at the shine. Truly glassy means obsidian — confirm with a curved fracture or a sharp translucent edge held to the light. Third, try to split it. Sheets mean shale — confirm with the penny scratch. Fourth, whatever is left — heavy, dull, hard, fine-grained — is basalt, and a scratch on glass seals it. The full background on each test is in the step-by-step identification guide and the Mohs hardness guide.

Two last cautions. Colour fools: weathered basalt can look brown, obsidian can be mahogany, and shale can be red or green, so always confirm with the tests rather than the tint. And coal country has one more black look-alike — slag, the glassy waste from smelting and railways, which is full of bubbles and often oddly coloured inside. If your "obsidian" came from a railroad bed or old industrial site and is full of frothy holes, suspect slag. Everyone else can file their find under one of the four, and move on to the next stone in the chart with confidence.