Rock identifier: reading minerals and formations
The physical properties geologists use to identify rocks and minerals — hardness, cleavage, lustre, and crystal form — explained for collectors.
The five properties that identify a mineral
Mineralogists identify unknowns through a sequence of physical tests rather than appearance alone — colour is the least reliable property because impurities shift it dramatically. The five most diagnostic properties are hardness, cleavage or fracture, lustre, streak, and crystal system. Hardness, measured against the Mohs scale from talc (1) at the softest to diamond (10) at the hardest, is quick to test with a fingernail (2.5), a copper coin (3.5), a steel blade (5.5), or a quartz crystal (7). Cleavage describes how a mineral breaks along flat planes determined by its crystal structure — perfect cleavage in three directions at right angles, as in halite, differs completely from the conchoidal (shell-like) fracture of obsidian. Lustre describes how the surface reflects light: metallic, vitreous (glassy), resinous, silky, or earthy. Together these properties narrow a specimen from thousands of known minerals to a handful of candidates.
Rock types and how they form
The three rock families — igneous, sedimentary, and metamorphic — each record a different chapter of geological history. Igneous rocks crystallise from cooling magma; intrusive types like granite cool slowly deep underground, producing coarse-grained textures with visible crystals, while extrusive types like basalt cool rapidly at the surface, yielding fine-grained or glassy textures. Sedimentary rocks form from the accumulation and compaction of particles derived from erosion of older rocks, biological material, or chemical precipitation from water. Their defining feature is layering (stratification), and they often contain fossils. Metamorphic rocks are existing rocks recrystallised under high pressure and temperature without melting — marble is metamorphosed limestone; slate is metamorphosed mudstone. Recognising which family a specimen belongs to immediately constrains what it can be and where it likely formed.
Using RockLens to accelerate field identification
RockLens analyses photographs of rock and mineral specimens to identify key visual characteristics — crystal form, surface texture, lustre, and colour distribution — and matches them against a database of documented species and varieties. The app returns a confidence-ranked list of candidates with the diagnostic features that drove each match. For rocks, it identifies the rock type (igneous, sedimentary, or metamorphic) and probable mineralogical composition. For minerals, it identifies the most likely species and common variety. Physical tests — hardness with a fingernail or coin, checking for cleavage by observing the broken surface, testing for effervescence with vinegar to confirm carbonate minerals — remain essential complements to the visual scan. The combination of RockLens's visual analysis and a brief physical test sequence produces reliable identifications for the vast majority of common rocks and minerals.
FAQ
Why is colour unreliable for identifying minerals?
The same mineral can occur in many different colours depending on trace impurities. Quartz, one of the most common minerals, occurs as colourless crystal, purple amethyst, pink rose quartz, grey smoky quartz, yellow citrine, and black morion — all the same mineral, SiO₂. Conversely, unrelated minerals can share similar colours. Streak — the colour of the mineral's powder scratched onto an unglazed ceramic tile — is far more consistent than surface colour and is one of the most reliable quick tests a collector can perform.
What is the difference between a rock and a mineral?
A mineral is a naturally occurring inorganic solid with a definite chemical composition and ordered atomic structure — quartz (SiO₂) and feldspar (various aluminium silicates) are examples. A rock is an aggregate of one or more minerals (or, in some cases, organic material). Granite is a rock composed predominantly of quartz, feldspar, and mica. Understanding this distinction matters for identification: you identify the rock type first (granite), then identify the individual mineral grains within it (quartz, K-feldspar, biotite) as a second step.
How do I photograph a specimen to get the best result from RockLens?
Place the specimen on a neutral background (white or grey paper) in natural daylight, avoiding direct sun which creates harsh shadows. Take one photo of the freshest surface (not weathered exterior crust if possible), one of a broken or cleaved face to show lustre and cleavage, and one showing the overall specimen shape. Include a coin or ruler for scale. Wet the surface lightly if needed — a small drop of water temporarily enhances lustre and colour. Avoid using flash, which bleaches colour and flattens texture.