Identify Minerals by Picture: What a Photo Can and Cannot Tell You

Start With What a Photograph Actually Records

A photograph records three things about a mineral: its colour, its shape, and the way light behaves at its surface. That is genuinely useful, because those three properties eliminate a large share of the candidates before you touch a test kit.

What a photograph cannot record is hardness, streak, specific gravity or reaction to acid. Those are the properties that confirm an identification, and they are why a photo answer is always a shortlist rather than a verdict.

The useful way to think about picture-based identification is this: the photo narrows the field, and two or three cheap bench tests finish the job. Treat any tool that skips the second half with suspicion.

The Five Properties Every Mineral Is Named By

Mineralogists describe a specimen with the same short list of properties every time. Knowing them is what turns a guess into a method.

• Colour — useful but unreliable. Quartz comes in almost every colour, and so does fluorite. Use colour to narrow, never to conclude.
• Hardness — the most diagnostic property you can measure at home, and the one that rules out the most candidates.
• Streak — the colour of the powder the mineral leaves on unglazed porcelain. Far more consistent than the colour of the specimen itself.
• Lustre — how the surface reflects light: metallic, glassy, waxy, pearly, silky or dull.
• Cleavage and fracture — whether it breaks along flat planes or in curved shells.

Hardness Without a Kit

The Mohs scale runs from 1 to 10, but you do not need the reference minerals. Four everyday objects cover almost everything you will pick up.

A fingernail is about 2.5. A copper coin is around 3.5. A steel knife blade is about 5.5, the same as window glass. A steel file is around 6.5.

• Scratched by a fingernail: below 2.5 — talc, gypsum, mica.
• Scratched by a coin but not a fingernail: around 3 to 3.5 — calcite.
• Scratched by a knife but not a coin: around 4 to 5 — fluorite, apatite.
• Scratches glass: above 5.5 — quartz, feldspar, garnet.
• Scratches a steel file: above 6.5 — beryl, corundum, topaz.

Streak: The Test Almost Everyone Skips

Streak is the colour of the mineral in powdered form, and it is far more reliable than the colour of the specimen. Hematite can be silver, red or black in the hand, but it always streaks reddish brown.

Rub the specimen across the unglazed back of a ceramic tile. If it is too hard to leave a mark, crush a small fragment on a white plate.

A few streaks are decisive on their own. Pyrite streaks greenish black, hematite red-brown, magnetite black and galena lead grey. Quartz and most silicates leave a white streak, which is itself informative.

Lustre, and Why the Surface Matters

Lustre describes how light leaves the surface, and it splits the common minerals cleanly. Metallic lustre looks like metal: pyrite, galena, magnetite, chalcopyrite. Submetallic sits between metal and glass and is typical of hematite.

Vitreous or glassy is the most common and covers quartz, calcite, fluorite and most gem minerals. Waxy covers the chalcedony family. Pearly turns up on mica, talc and some feldspars. Silky appears on fibrous minerals such as gypsum. Dull or earthy covers the clays and limonite.

If your specimen is metallic, the field of candidates drops to about a dozen minerals, and streak plus hardness will separate them.

Photographing a Mineral So the Answer Is Usable

Most failed identifications are photography failures rather than tool failures.

1. Shoot in daylight, out of direct sun, with no flash. Flash flattens lustre and blows out the surface.
2. Take one shot of the whole specimen so habit and scale are visible, then a close-up of a broken face rather than a weathered one.
3. Add something for scale — a coin is fine. Crystal size and habit are diagnostic.
4. Take a second angle. Cleavage planes and crystal form only show from certain directions.
5. Wet the surface for one shot only. It reveals colour and transparency but hides texture, so keep a dry shot too.

The Bottom Line

A photo gives you colour, habit and lustre, and that is enough to reach a shortlist of three or four minerals. Hardness and streak take you the rest of the way.

Do not trust a single photograph on its own, and be sceptical of any identification that comes without a test you can run yourself. The point of the exercise is a name you can defend.

For the method in more detail, see mineral identifier and rock identification by photo. Crystal identification app covers the crystal case, and pyrite identifier and mica identifier show streak and cleavage working in practice.