Mineral identification key: one property at a time

Tilt a fresh, dry surface under a lamp. A metallic mineral is opaque even in thin slivers and reflects like polished or tarnished metal: brassy pyrite, lead-grey galena, black magnetite, steel-grey hematite. Everything else is non-metallic, and the kind matters: glassy like quartz, pearly like a mica face, greasy like talc, dull like earthy hematite. A photo in angled light settles this step if the specimen is dry; water fakes a glassy shine.

Color is the least reliable property for non-metallic minerals: quartz, calcite and fluorite each come in many colors, so record it but do not decide on it. For metallic minerals the powder color, the streak, is diagnostic. Rub an inconspicuous corner of a common specimen on the unglazed back of a white tile: hematite leaves red-brown whatever its surface color, pyrite greenish black, magnetite black, galena lead grey.

Streak needs that tile; a finger, paper or a photo cannot give it. A mineral harder than the tile, about Mohs 6.5, leaves no streak and scratches the porcelain instead. Never streak a polished piece, a crystal cluster or anything of possible value.

Work up the ladder from the softest tool: a fingernail is about 2.5, a clean copper coin about 3, a steel nail or a glass plate about 5.5. Press the tool on an inconspicuous spot of a common specimen, wipe it, and check with a lens for a groove rather than a metal smear. The first tool that truly scratches brackets the hardness: fingernail marks talc and gypsum, coin marks calcite, nail marks fluorite and feldspar, and nothing household marks quartz or garnet.

Mohs numbers are rankings, not measurements, and each mineral sits in a range, so use the ladder to bracket rather than pin. Do no scratch test on a gem, a cut stone or a specimen you may sell.

Look at every broken edge, not the crystal faces. Cleavage shows as flat, mirror-like planes that repeat in steps and flash together when you tilt the specimen. Count the directions and judge the angle: one direction gives sheets (mica); two at right angles gives blocky steps (feldspar); two at about 60 and 120 degrees gives splinters (hornblende); three at right angles gives cubes (galena); three oblique gives rhombs (calcite); four gives octahedral corners (fluorite).

No flat planes means fracture. Curved, shell-like conchoidal surfaces with fine ribs are typical of quartz, garnet and olivine; uneven fracture is common in pyrite. Lit from the side, a photo shows stepped cleavage and conchoidal ribs well enough for the tool to use.

If any crystal faces survive, name the shape: striated cubes for pyrite, cubes for galena and fluorite, six-sided prisms for quartz, rhombs for calcite, flat books for mica, twelve-sided balls for garnet, bladed crystals for gypsum, long prisms for hornblende. Habit photographs well and is the main thing the tool reads beyond luster.

Then weigh the specimen in your hand against a quartz pebble of the same size. Galena feels nearly three times as heavy, magnetite, hematite and pyrite about twice, while most pale minerals feel like quartz. A fridge magnet that clings firmly points to magnetite. Heft and magnetism do not show in any photo.

The specimen is pale pink and blocky with a glassy to slightly pearly shine, so couplet 1 goes to 3. A fingernail and a coin leave nothing and a steel nail skates off, so 3b goes to 5. The broken corner shows two sets of flat faces meeting at about a right angle, stepping down like a staircase, so 5a goes to 6, and two cleavages at right angles in a pink blocky habit is potassium feldspar.

Had the same pink specimen been rounded and greasy-glassy, with curved conchoidal breaks and no flat planes, 5b would have ended at rose quartz. That difference, stepped planes versus shell-like curves, is what to photograph from the side.

Typical hardness is a range to bracket with, not to pin. The last column is what Rock Identify can read from one to three JPG, PNG, WebP or HEIC photos, up to 40 MB each, before returning three candidates with the visible evidence for each and taking a follow-up question; three identifications a day are free without an account.