Identify Rocks & Minerals,
Field Tests that Never Lie.
A rock and mineral identification guide: 20+ common minerals and rocks with hardness, streak, luster, and the field tests that distinguish lookalikes.
Mohs Hardness Scratch Test
Fingernail (2.5), copper penny (3.5), steel knife (5.5), and glass plate (6.5) field benchmarks.
Streak Plate Pigment
Scratching unglazed porcelain reveals true mineral powdered pigment regardless of surface tarnish.
Cleavage vs Conchoidal Fracture
Distinguish geometric planar sheets (mica/calcite) from curved shell-like breaks (obsidian/quartz).
RockIDGuide Profiles
Quartz
The hardest common mineral — glassy, colorless to colored, and the standard every scratch test calibrates against.
Feldspar (Orthoclase & Plagioclase)
The most abundant mineral family — pink, white, or gray blocks with two cleavages at right angles.
Calcite
The acid-fizzing master — rhombohedral cleavage, hardness 3, and the mineral marble and limestone are made of.
Pyrite ("Fool's Gold")
The brassy impostor — metallic gold color, iron-hard, and cubic crystals that fooled every gold rush.
Mica (Muscovite & Biotite)
The peelable mineral — perfect sheet cleavage into transparent or black elastic foils.
Garnet
The rounded red crystals in schist — the abrasive mineral hiding in metamorphic rock.
Halite (Rock Salt)
The taste-test mineral — cubic crystals, salty, and the only mineral you are allowed to lick.
Gypsum (Selenite)
The fingernail-soft crystal — transparent blades and the mineral inside every wall.
Magnetite
The magnetic iron ore — black, metallic, and the magnet test that never lies.
Hematite
The red-streak iron mineral — metallic silver-gray or earthy red, named for its blood-red streak.
Olivine (Peridot)
The olive-green mantle mineral — glassy green grains from the deep earth and the beach sands of Hawaii.
Talc
The softest mineral — soapy, greasy, and the reference point for "1" on the hardness scale.
Fluorite
The rainbow mineral — purple, green, blue, and yellow cubes that glow under UV light.
Amethyst
The purple quartz — geode linings and crystal clusters, colored by iron and ancient radiation.
Granite
The kitchen-counter classic — interlocking quartz, feldspar, and mica cooled slowly underground.
Basalt
The ocean-floor rock — fine-grained, dark, and covering most of the planet underfoot.
Limestone
The fizzing sedimentary rock — calcium carbonate from ancient seas, full of fossils.
Sandstone
The lithified beach — sandy grit, layered colors, and the rock that records ancient rivers and dunes.
Shale
The mud-rock — thin, layered sheets that split along bedding and smell earthy when breathed on.
Slate
Shale all grown up — harder, denser, ringing when tapped, and splitting into roof-perfect sheets.
Marble
Limestone recrystallized — sugary sparkle, no fossils left, and still fizzing in acid.
Quartzite
Sandstone fused solid — harder than glass, sparkly with fused grains, and nearly unbreakable.
Field Research & Methods
The field kit: what to carry and what the tests actually do
Hardness picks, streak plate, acid bottle, magnet — the four tools that identify ninety percent of finds.
Where to actually find rocks: reading the landscape
River gravels, road cuts, construction sites, and beaches — the honest geography of finds.