Free Rock Identifier by Photo —
What Is My Rock?

How to Identify Rocks — What the AI Looks For

When you upload a photo to our free rock identifier, the AI analyses the same visual properties a geologist would examine in the field. Understanding what these properties are helps you take better photos and interpret your results more confidently.

Colour and Colour Pattern

Colour is one of the first things we notice about a rock, but it is not the most reliable identifier on its own — many different rocks share similar colours. However, colour pattern is very informative: banding, colour zoning, speckles, or a uniform single colour all narrow down the possibilities significantly. Note whether the colour is uniform or varied, and whether any variation follows a pattern.

Texture and Grain Size

Texture refers to the size, shape, and arrangement of the mineral grains or crystals that make up the rock. Coarse-grained rocks (individual crystals visible to the naked eye, larger than 2mm) suggest slow cooling underground — granite is the classic example. Fine-grained rocks (grains too small to see without magnification) suggest rapid cooling at the surface — basalt is typical. Glassy rocks with no grains suggest extremely rapid cooling — obsidian forms this way.

Crystal Structure and Habit

Many rocks and minerals form distinctive crystals with recognisable shapes: hexagonal prisms (quartz), cubic crystals (pyrite, halite), rhombohedral cleavage (calcite). The crystal habit is often visible in a close-up photograph and provides strong identification clues. Our AI is trained to recognise crystal faces, cleavage planes, and habit patterns from photos.

Luster

Luster describes how light reflects from a surface. Main categories: metallic (like polished metal — pyrite, galena), vitreous or glassy (like glass — quartz, obsidian), pearly (soft iridescent sheen — some micas), silky (parallel fibre sheen — satin spar gypsum), earthy or dull (no reflectivity — clay minerals). Luster is best assessed with the rock dry and clean in natural light.

Rock Identification Field Tests You Can Do at Home

While our AI works from photos alone, providing physical test results in the notes field significantly improves your identification. These simple tests take seconds and require no specialist equipment.

The Scratch Test (Mohs Hardness)

The Mohs scale rates mineral hardness from 1 (talc) to 10 (diamond). Everyday reference points: your fingernail is approximately Mohs 2.5, a copper coin is Mohs 3.5, a steel knife blade is Mohs 5.5, and glass is Mohs 5.5–6. If a steel knife scratches your rock easily, it is below Mohs 5.5. If your rock scratches glass, it is above Mohs 5.5. These two data points immediately narrow down the possibilities considerably.

The Acid Test

A drop of white vinegar causes immediate fizzing on any carbonate mineral — limestone, chalk, calcite, and marble all react visibly. Dolomite reacts only when scratched or powdered. This test definitively confirms or eliminates carbonate rocks in 10 seconds. Always test an inconspicuous spot as acid can etch polished surfaces.

The Magnet Test

Iron-bearing minerals — particularly magnetite and some pyrrhotite — are strongly magnetic. If your rock attracts a magnet, this significantly narrows the possibilities. Iron meteorites are typically strongly magnetic; some basalts show weak attraction from magnetite content. Most common rocks are non-magnetic.

The Streak Test

Rubbing a mineral across unglazed porcelain (the unfinished back of a ceramic tile) leaves a coloured powder that is often different from the surface colour and far more consistent. Pyrite (fool’s gold) leaves a black streak despite its golden colour; real gold leaves a golden streak. Hematite leaves a distinctive red-brown streak despite often appearing metallic grey. Always mention streak colour in your notes.

Common Rocks and How to Identify Them

These are the rocks people most frequently upload to our identifier. Understanding their key features helps you provide better information and interpret your result.

Granite

One of the most common continental crust rocks — a coarse-grained igneous rock with clearly visible interlocking crystals of quartz (grey, glassy), feldspar (white, pink, or cream), and mica (black biotite or silver muscovite). Very hard (Mohs 6–7), heavy, and does not react to acid. The classic speckled “worktop stone.”

Limestone

A sedimentary rock made primarily of calcium carbonate — from marine shells or chemical precipitation. White, cream, yellow, or grey. Relatively soft (Mohs 3). Key test: fizzes immediately with vinegar. Often contains fossils. Fine-grained varieties look uniform; some show visible shell fragments.

Sandstone

A sedimentary rock of sand-sized grains cemented together. Colour varies enormously — red/orange from iron oxide, white from pure quartz, yellow-brown from iron hydroxide. Has a characteristic gritty feel and visible grain texture. May show cross-bedding or lamination. Use our stone identifier for sandstone variants.

Basalt

A dark, fine-grained igneous rock — the most abundant rock type on Earth’s oceanic crust and volcanic islands. Dark grey to black, dense, and heavy. Individual grains are too small to see without magnification. May contain small round holes (vesicles) where gas bubbles were trapped. Often weakly magnetic from magnetite content.

Quartz

The most common mineral in Earth’s continental crust. Pure quartz is colourless; impurities produce purple (amethyst), yellow-orange (citrine), pink (rose quartz), and grey-black (smoky quartz). Mohs 7 — scratches glass easily. Conchoidal fracture. No cleavage. Does not react to acid.

Where to Find Interesting Rocks

You don’t need to be in a famous rockhounding location to find interesting specimens. Here are the best places to look:

• Riverbeds and streams — water erosion exposes a wide variety of rock types. Harder minerals concentrate naturally as softer ones are removed.
• Road cuts and construction sites — fresh rock faces that haven’t been weathered. Some of the best geological exposures are along highway cuts.
• Beaches and coastal cliffs — wave action delivers rocks from a wide catchment. Pebble beaches are essentially a free geological survey of local bedrock.
• Old mine dumps and quarry spoil — waste rock from historical mining often contains visually striking specimens discarded as commercially unimportant.
• Ploughed fields — frost action and ploughing bring subsurface material to the surface. Fields near chalk bedrock produce flints; granite areas expose quartz veins.
• Mountain and upland paths — erosion and trail wear constantly expose fresh rock. Upland areas have more interesting geology than lowlands due to less soil cover.

Using Our Specialist Identifier Tools

The general rock identifier works for any rock or mineral. But if you already suspect what you have, our specialist tools give more targeted results with AI prompts specifically tuned to that material’s identifying features. For gemstone identification, our gemstone identifier and crystal identifier assess colour quality, clarity, likely treatment, and collector value. For the gold vs pyrite question we get asked constantly, our gold identifier specifically distinguishes genuine gold from fool’s gold. For potential space rocks, our meteorite identifier looks for fusion crust, Widmanstätten pattern, chondrules, and regmaglypts.