How to identify rocks with one specimen on the table

Scrub the specimen with water and an old toothbrush, then let it dry. Dust and lichen hide grain boundaries, and a wet surface fakes a glassy luster that vanishes once the rock is dry. If orange or brown color rinses off, that was iron staining, not the rock's own color.

Turn the rock until you find a chip, corner or crack where the interior shows. The weathering rind is often a few millimeters of altered material that looks nothing like the fresh inside, so judge every step below on the fresh face. Never hammer a possible gem, fossil or anything you might sell; with no broken face, photograph the cleanest corner and note it.

Hold the fresh face a hand's width from your eye in good light. If you can pick out separate grains wider than the ridge of a fingernail, about a millimeter and up, the rock is coarse grained. If you see a texture but cannot separate grains, it is fine grained. A smooth, glassy or waxy surface with no grain at all is worth writing down too.

Then ask how the grains meet. Interlocking crystals fit like puzzle pieces with no space between them and flash as small flat faces when you tilt the rock. Rounded or angular grains in a finer matrix, like sand in cement, were deposited and glued. Grains flattened or lined up in one direction have been squeezed.

Flat, parallel beds that run through the whole rock, sometimes a shade different in color or grain size, point to sediment. Light and dark bands that wrap, fold or pinch out, with crystals visible inside each band, point to a metamorphic rock, as does a rock that splits into thin sheets along one direction.

Round or slightly stretched holes were gas bubbles, called vesicles, and they mean lava. Dense dark rock with pinhead vesicles is usually basalt; a pale frothy rock that floats is pumice; a red or black rock full of large rough-walled holes is scoria. Glass with curved, shell-like fracture and sharp edges is obsidian, or chert if waxy and grey, brown or red.

Tilt the face under a lamp and watch how it reflects. Glassy flashes from flat crystal faces, a pearly sheen along a split surface, a metallic glint, or a dull earthy look are different lusters, and a photo taken at the same angle records them well. Sparkle from hundreds of tiny flakes that shift as you turn the rock is almost always mica.

Hold a thin edge up to daylight. Quartz, calcite and agate let light through at the edges; nearly all whole rocks stay opaque. Then weigh the specimen in your hand against a known rock of similar size and write heavy, normal or light. Heft is a hint, not a measurement, and no photo can read it.

On a specimen you are sure is an ordinary rock, choose a spot on the back and try, in order, your fingernail, a copper coin and the point of a steel nail. A fingernail marks talc and gypsum, a coin marks calcite, and a steel nail marks most minerals up to feldspar but is stopped by quartz. Wipe the spot and look with a lens; a soft tool can leave a metal smear that is not a scratch.

This is a rough check, not a Mohs test; never do it on a polished stone, a crystal cluster, a fossil or anything that may have value, because the mark is permanent. A photo cannot show hardness either, so your scratch note is a check you add to the tool's candidates.

Put the notes together. Interlocking crystals with no layers, or glass and vesicles, mean igneous. Cemented grains, fragments, shells or flat beds mean sedimentary. Folded bands, foliation or stretched grains mean metamorphic. One consistent color, luster and crystal shape throughout, often with flat faces or clean breaks, means a single mineral rather than a rock. Even color, bubbles in a glassy mass or a flat poured base point to slag, brick or glass.

Only now go to a candidate. Coarse, light, speckled and igneous is probably granite; fine, dark and vesicular is basalt; sand grains cemented in beds is sandstone; coarse crystals in folded bands is gneiss. Treat each as a hypothesis and name the feature that would rule it out.

Take one photo of the whole rock on a plain background, one close-up of the fresh face in angled light, and one of the most telling feature. Rock Identify accepts one to three JPG, PNG, WebP or HEIC photos of the same specimen, up to 40 MB each, and returns up to three candidates with the visible evidence for each; three identifications a day are free without an account. If its first candidate disagrees with your group decision, ask in the follow-up which feature it weighted, then check that feature yourself.