Seven short guides that cover the tests real rockhounds use. Pair them with a photo identification for a much more confident result.
Hardness is the single most useful field test. A fingernail scratches about 2.5, a copper coin about 3.5, a steel knife or nail about 5.5, and window glass about 5.5 too. If your stone scratches glass it is harder than 5.5 — that already rules out calcite, gypsum and most soft carbonates. Always test on an inconspicuous edge, and scrub away the mark to check whether you scratched the stone or just left a metal streak on it.
Rub the stone across unglazed porcelain. The powder it leaves is the streak, and it is far more reliable than surface colour. Hematite looks metallic grey but streaks red-brown. Pyrite (fool's gold) streaks greenish-black, while real gold streaks gold. Minerals harder than the porcelain (about 7) will not streak at all — that is information too.
Lustre describes how a surface reflects light: vitreous (glassy, like quartz), metallic (pyrite, galena), resinous (amber, sphalerite), pearly (mica, talc), silky (satin spar gypsum) or earthy (kaolinite). Combine lustre with transparency — transparent, translucent or opaque — and you have narrowed the field enormously before you even consider colour.
Habit is the shape a mineral naturally grows into: quartz forms six-sided prisms with pointed terminations, pyrite grows cubes, fluorite grows cubes and octahedra, and calcite forms rhombs. Cleavage is how it breaks — calcite splits into perfect rhombs, mica peels into sheets, and quartz has no cleavage at all, breaking with curved conchoidal fractures instead.
This is the most common beginner mix-up. Quartz is hardness 7, will not fizz in vinegar and shows conchoidal fracture. Calcite is hardness 3, fizzes in dilute acid and cleaves into rhombs. Manufactured glass has hardness around 5.5, often contains round bubbles, and its edges may show mould seams. If the piece is suspiciously flawless and vividly coloured, suspect glass or dyed agate.
Use indirect daylight rather than a flash, put the specimen on a plain neutral background, fill the frame with one stone, and hold the camera steady with the focus locked on the surface texture. If the stone is translucent, take a second photo backlit. Wet the surface lightly to bring out colour banding — just make sure the stone is water-safe first.
Warm water, mild soap and a soft brush handles most quartz-family stones. Never soak selenite, halite, or anything fibrous — they dissolve or fray. Malachite, turquoise and pyrite dislike acids and prolonged moisture. Amethyst, rose quartz, celestite and fluorite fade in direct sunlight, so display them away from windows.
