Mineral identifier: the field collector's systematic approach
From hardness tests to cleavage angles — how to build a reliable mineral identification workflow wherever you are collecting.
The Mohs hardness scale in practice
Friedrich Mohs designed his relative hardness scale in 1812 using ten reference minerals, each capable of scratching all those below it. The scale runs from talc (1) — scratched by a fingernail — through gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase feldspar (6), quartz (7), topaz (8), corundum (9), to diamond (10) at the hardest. In the field, a standard kit of fingernail, copper coin, steel blade, and quartz crystal gives you reference points at 2.5, 3.5, 5.5, and 7. Testing hardness takes thirty seconds: scratch the mineral against the reference, then blow away the powder. If the mineral is scratched, it is softer; if the reference is scratched, the mineral is harder. Most common rock-forming minerals cluster between 5.5 and 7.5 — quartz, feldspar, and pyroxene — while economically significant sulfide minerals like pyrite and galena fall below 6. Hardness alone often narrows an unknown to three or four candidates.
Cleavage, fracture, and what they reveal about crystal structure
When a mineral breaks, it breaks either along planes of weak atomic bonding (cleavage) or in an irregular pattern (fracture). Cleavage planes are diagnostic because they are a direct expression of the mineral's crystal structure. Mica breaks into perfect thin sheets — one direction of perfect cleavage — because its sheet-silicate structure has one plane of weak bonds. Halite (rock salt) breaks into perfect cubes — three directions of cleavage at right angles — because the ionic bonds in all three cubic directions are equal. Amphiboles and pyroxenes are distinguished by the angle between their two cleavage directions: amphiboles intersect at 60° and 120°; pyroxenes at 90°. This single measurement under a hand lens separates two otherwise similar-looking mineral groups. Fracture types (conchoidal in glass and quartz, hackly in native metals, earthy in kaolinite) add additional discriminating information for minerals without well-developed cleavage.
Building a reference collection for comparative identification
The fastest path to confident mineral identification is direct comparison against labelled reference specimens. A compact personal reference collection of twenty to thirty common and locally relevant minerals — each with a fresh surface, labelled with species, locality, and hardness — gives you immediate comparison material in the field and at the display table. Start with the ten Mohs reference minerals plus common accessory minerals like magnetite (magnetic), calcite (effervesces in vinegar), galena (high density, cubic cleavage), and sulfur (yellow, very low hardness). Each new acquisition becomes a comparison benchmark for future unknowns. RockLens's scan history functions as a visual reference library in the same way: every confirmed identification you add enriches the personal database you are building of the minerals in your collecting area.
FAQ
What is streak and why is it more reliable than surface colour?
Streak is the colour of a mineral's powder produced by scratching it across an unglazed porcelain streak plate (hardness about 6.5). Because the powder eliminates the surface effects that alter apparent colour — reflective coatings, surface oxidation, light interference — the streak colour reflects the mineral's intrinsic pigmentation. Pyrite (fool's gold) has a brassy metallic surface but leaves a greenish-black streak; gold leaves a golden-yellow streak identical to its surface colour. Streak plates are inexpensive and fit in a pocket — one of the most cost-effective identification tools available to a field collector.
What makes pyrite look like gold and how do you tell them apart?
Pyrite (iron sulfide, FeS₂) has a brassy-yellow metallic lustre that closely resembles gold at first glance. The simplest tests separate them immediately. Hardness: pyrite is hard (6–6.5) and cannot be scratched by a fingernail or copper coin; gold is soft (2.5–3) and easily deformed by pressure. Streak: pyrite's streak is greenish-black; gold's streak is golden-yellow. Crystal form: pyrite often forms perfect cubic or pyritohedral crystals with striated faces; gold typically forms rounded nuggets, wires, or flakes. Density: gold (19.3 g/cm³) is dramatically denser than pyrite (5.0 g/cm³) — a genuine gold nugget feels unmistakably heavy for its size.
How do I know if a mineral specimen is worth collecting?
Value in mineral collecting is multi-dimensional. Scientific value depends on locality specificity, rarity, and quality of crystallisation. Aesthetic value depends on crystal size, form, colour, and matrix presentation. Educational value depends on how well the specimen displays the diagnostic features of the species. A perfectly formed small crystal on a clean matrix often has more collector value than a large but poorly formed mass of the same species. Documenting the exact locality (GPS coordinates and geological context) when you collect increases the scientific value of any specimen significantly, because provenance is one of the key factors experienced collectors and institutions consider.