Mineral Quick-ID: scratch test, streak, and luster
Three questions a collector can answer standing over the outcrop — what is the softest thing that scratches it, how does it catch the light, and what color does it leave on a streak plate — narrow a mystery rock to its likely candidates. The full reference sits below the questions, and the field card at the bottom prints.
Likely candidates
Answer a question above and the candidates appear here. Every mineral is in the table below.
Hardness, streak, and luster for 38 common finds
The whole drawer at once. The last column is the one detail that settles each call in the field — the test or the feature you can check on the spot.
| Mineral | Mohs | Streak | Luster | The tell |
|---|---|---|---|---|
| Quartz (Rock Crystal) | 7 | white | vitreous | Glass-clear six-sided prisms that cut a knife blade and refuse to be scratched back, with no cleavage at all — break one and you get curved, shell-like surfaces instead of flat faces. Calcite is the usual mix-up, and it is far softer and fizzes in vinegar. |
| Amethyst variety of quartz | 7 | white | vitreous | Purple quartz, and the purple is almost never even — look for color pooled in the top third of each crystal or stacked in angular zones. Fluorite comes in the same purple but a steel knife scratches it easily; amethyst will not budge. |
| Smoky Quartz variety of quartz | 7 | white | vitreous | Transparent brown-to-black quartz that still shows the six-sided prism and glassy shine; hold it to the sun and light gets through, which separates it fast from opaque black obsidian or schorl. Hardness above 7 and a white streak seal it. |
| Rose Quartz variety of quartz | 7 | white | vitreous | Cloudy translucent pink that almost never shows a crystal face — rose quartz turns up as massive chunks in pegmatite, not as points. Pink calcite and rhodochrosite are the common confusions and both are soft enough to knife; rose quartz shrugs a blade off. |
| Chalcedony variety of quartz | 6.5–7 | white | waxy | Quartz too fine-grained to show any crystal: the surface looks like frozen candle wax or a bunch of grapes, and a fresh break is smooth and faintly greasy. Put a flashlight behind a thin edge and it glows translucent, which rules out opaque jasper. |
| Agate variety of chalcedony | 6.5–7 | white | waxy | Banded chalcedony — the giveaway is curved, concentric layering that follows the shape of the cavity the agate grew in, not the straight bedding of a sedimentary rock. Wet the cut face and the bands sharpen while the translucent zones light up. |
| Jasper variety of chalcedony | 6.5–7 | white | dull | Opaque even on a thin edge held to a flashlight — that is the working line between jasper and agate or chalcedony, which both pass light. It still cuts glass and polishes beautifully; the color comes from iron and clay packed into the silica. |
| Flint / Chert variety of chalcedony | 6.5–7 | white | glassy-to-waxy | Dull and chalky on the outside rind but glassy-waxy on a fresh break, with sharp conchoidal fracture that leaves edges you can cut yourself on. Chert is the general rock; flint is the darker, finer chert that knaps cleanly. |
| Common Opal variety of opal | 5.5–6.5 | white | waxy | Opal never grows a crystal — it is silica with water still locked inside, so look for smooth, waxy, almost plastic-looking masses. A quartz point will scratch it, and common opal shows no flashes of color no matter how you tilt it. |
| Precious Opal variety of opal | 5.5–6.5 | white | vitreous | Rock it under a light and patches of pure spectral color slide across the stone — nothing else common does that, and it is called play-of-color. Color that stays put as you tilt the piece is only body color, not fire. |
| Obsidian | 5–6 | white | vitreous | Volcanic glass: jet black, glossy, completely without grain or crystal faces, and it breaks into razor conchoidal flakes. Black tourmaline and smoky quartz look close until you notice obsidian has no faces at all, and thin edges glow brown against a bright light. |
| Petrified Wood | 6.5–7 | white | waxy | Stone that kept the wood: look for growth rings, grain running one direction, and knot or bark texture on a piece hard enough to scratch glass. If a fingernail marks it or it crumbles at the edges, you are holding a wood-shaped sandstone instead. |
| Geode | 3–7 | white | vitreous | A knobby, unimpressive ball that feels too light for its size — hollow inside, lined with quartz or calcite points growing toward the center. Tap two together in the field: a solid nodule thuds, a geode gives a hollow knock. |
| Calcite | 3 | white | vitreous | A drop of dilute acid or plain vinegar sets it fizzing, and that test is close to conclusive on its own. Backing it up: a copper penny scratches calcite, and it cleaves into leaning rhombs where quartz breaks in curved shells. |
| Fluorite | 4 | white | vitreous | Candy-colored cubes, frequently with two or three colors zoned into a single crystal. A steel knife scratches it without effort, and it cleaves on four octahedral directions so knocked corners come off as neat little triangles — something quartz never does. |
| Microcline (Amazonite) variety of feldspar | 6–6.5 | white | vitreous | Feldspar breaks along two flat reflective planes that meet at about 90 degrees, so a broken face flashes like a small mirror as you rotate it in the sun; quartz has no such planes. Blue-green amazonite is microcline, and the fine white streaking along its cleavage is the second tell. |
| Almandine Garnet variety of garnet | 7–7.5 | white | vitreous | Round, many-faced red crystals sitting in schist or gneiss with no cleavage whatsoever — garnet is the ball-shaped mineral in metamorphic rock. It scratches quartz, feels heavy for its size, and still leaves a white streak despite the dark red body color. |
| Pyrope Garnet variety of garnet | 7–7.5 | white | vitreous | Pyrope runs a clearer, more fiery red than almandine's brownish tone, and it turns up as loose stream-worn grains and anthill pickings rather than embedded in schist. Both scratch quartz and streak white, so color and setting are what separate them in the field. |
| Topaz | 8 | white | vitreous | It feels startlingly heavy for a pale glassy crystal, and it carries one perfect cleavage straight across the prism — set a topaz down hard and a flat lid can pop off the end. The grooves run the length of the crystal, where quartz is grooved crosswise. |
| Ruby variety of corundum | 9 | white | adamantine | Red corundum, and hardness 9 means it scratches topaz and every quartz in your bag while nothing in a normal collection scratches back. Look for a stubby six-sided barrel with flat, often stepped ends and a heft well above its size. |
| Sapphire variety of corundum | 9 | white | adamantine | Corundum in any color except red. In stream gravel it shows up as heavy, rounded, six-sided pebbles with a soapy-smooth surface; touch a topaz point to it — sapphire scratches topaz, and nothing else in your bucket will. |
| Emerald variety of beryl | 7.5–8 | white | vitreous | Green beryl colored by chromium — the field mark is a flat-ended six-sided prism in mica schist or pegmatite, harder than quartz. Almost every emerald is threaded with internal fractures, so a big flawless green crystal in the field deserves suspicion. |
| Aquamarine variety of beryl | 7.5–8 | white | vitreous | Long, glassy, flat-ended six-sided prisms out of pegmatite pockets — that flat termination is the quick split from quartz, which comes to a point. It also scratches quartz and holds a cleaner, bluer color than pale topaz. |
| Schorl (Black Tourmaline) variety of tourmaline | 7–7.5 | white | vitreous | Run a fingernail across it: tourmaline is grooved lengthwise like a bundle of straws, and the crystal's cross-section is a rounded triangle rather than a hexagon. Black and glassy, it scratches quartz, and unlike obsidian it shows genuine crystal faces. |
| Elbaite (Colored Tourmaline) variety of tourmaline | 7–7.5 | white | vitreous | Same grooved prism and rounded-triangle cross-section as schorl, but in color — and frequently several colors in one crystal, switching along the length or from a pink core to a green rind. Harder than quartz with lengthwise striations, which rules out colored fluorite and calcite. |
| Pyrite | 6–6.5 | greenish-black | metallic | Brass-yellow metal in hard striated cubes, and the streak plate settles the argument: pyrite smears greenish-black while gold smears golden yellow. Pyrite is also brittle — press a knife point in and it crumbles, where gold dents and flattens. |
| Galena | 2.5 | lead-gray | metallic | Absurdly heavy for its size, and it cleaves in three directions at right angles — tap a piece and you get a scatter of tiny silver boxes. It is soft enough to mark with a copper penny, which nothing else bright and metallic in the field will be. |
| Magnetite | 5.5–6.5 | black | metallic | Put a magnet on it. Magnetite is the one common mineral that snaps to a magnet on its own and does it strongly, and a black streak plus black octahedral crystals confirm the call. Hematite is the frequent mix-up and streaks red-brown instead. |
| Hematite | 5–6.5 | red-brown | metallic | Whatever color the specimen is — mirror silver, black, or brick red — the streak comes out red-brown every time, and that one test names it. Magnetite looks nearly identical in hand but streaks black and grabs a magnet, which hematite generally will not. |
| Malachite | 3.5–4 | pale green | silky | Vivid green with light-and-dark concentric banding, soft enough for a knife, and a drop of dilute acid sets it fizzing because it is a carbonate. The pale green streak is what separates it from harder green crusts like chrysocolla. |
| Azurite | 3.5–4 | light blue | vitreous | A blue so saturated it looks painted on, riding weathered copper ore and almost always in company with green malachite. It is soft, gives a light blue streak, and fizzes in dilute acid — which clears out every hard blue lookalike at once. |
| Turquoise | 5–6 | pale bluish-white | waxy | Opaque robin's-egg blue with a waxy sheen and usually a web of brown or black host rock running through it. It is soft enough that the low end will not scratch glass, and it never fizzes, which sorts it from azurite and malachite crusts. |
| Gypsum (Selenite) | 2 | white | pearly | A fingernail scratches it, which by itself eliminates nearly everything else clear and glassy in the field. Selenite splits into thin sheets that bend without springing back, and it stays quiet under acid where calcite fizzes. |
| Muscovite (White Mica) | 2–2.5 | white | pearly | One perfect cleavage: peel it with a fingernail and you get springy transparent sheets you can read through. Green chlorite peels the same way but its flakes stay bent — muscovite's snap back flat. |
| Native Copper | 2.5–3 | copper-red | metallic | Scrape through the green or black skin and you hit bright salmon-pink metal that bends rather than breaks — copper is malleable, so a hammer flattens it instead of shattering it. Nothing else in the field is both copper-colored and bendable. |
| Native Gold | 2.5–3 | golden yellow | metallic | Gold stays bright yellow in the shade and inside a shadow, while pyrite and mica flash bright then go dark as you tilt them. It is soft, heavy and malleable — press a flake with a knife point and it dents, where pyrite crumbles and mica shatters. |
| Staurolite | 7–7.5 | white | resinous | Dull, dark brown prisms with a resinous rather than glassy shine, sitting in mica schist — and the crystals often grow through each other at right angles or at sixty degrees to make the famous cross. Harder than quartz, and it still streaks white despite the dark body color. |
| Sodalite | 5.5–6 | white | vitreous | Deep royal blue cut by white veining, softer than quartz, and it leaves a white streak — a white streak off a strongly blue stone is the fast check. Lapis lazuli looks close but carries brassy pyrite flecks that sodalite does not. |
Hardness, streak, and luster follow standard mineralogy references — Mindat and the conventions of Klein & Hurlbut’s Manual of Mineralogy — simplified into tests a collector can run with a fingernail, a penny, a knife, and a streak plate.
Prints on its own at half-letter size — fold it into a field bag.
Mohs hardness — field card
RockhoundDates · the scratch ladder and what you can carry to test it
| Mohs | Reference mineral | What you can carry instead |
|---|---|---|
| 1 | talc | — |
| 2 | gypsum | fingernail (2.5) |
| 3 | calcite | copper penny (3.5) |
| 4 | fluorite | — |
| 5 | apatite | steel knife blade and window glass (5.5) |
| 6 | orthoclase feldspar | steel file and unglazed streak plate (6.5) |
| 7 | quartz | quartz pebble (7) |
| 8 | topaz | — |
| 9 | corundum | — |
| 10 | diamond | — |
The scratch test, in order
- A fingernail scratches it — 2.5 or softer. Selenite, mica, talc.
- A penny scratches it, a fingernail does not — up to 3.5. Calcite, galena, gold, native copper.
- A steel knife scratches it, a penny does not — up to 5.5. Fluorite, malachite, azurite, turquoise.
- It scratches window glass — 5.5 and up; if it also scratches a quartz pebble, 7 and up. Quartz, garnet, tourmaline, topaz, corundum.
Streak: unglazed porcelain, read the powder. Harder than the plate leaves none — count it white.
Questions
What is the Mohs scale?
A ladder of ten reference minerals, talc at 1 up to diamond at 10, ordered by which one scratches which — a mineral higher on the ladder marks everything below it. It measures scratch resistance only, not toughness: diamond sits at 10 and still splits cleanly if you strike it wrong. The steps are nowhere near even, and the jump from corundum at 9 to diamond at 10 is wider than 1 through 9 put together.
How do I do a streak test?
Drag the specimen firmly across a piece of unglazed porcelain — a proper streak plate, or the unfinished back of a bathroom tile — and read the color of the powder it leaves, not the mark itself. That powder is the mineral’s true color and it ignores whatever the surface is doing: hematite turns up mirror-silver, black, or brick red in the hand and streaks red-brown every time. The plate is about 6.5 on the Mohs scale, so anything harder — quartz and up — scratches it instead of powdering, and that counts as a white streak.