Orvik or Rock Identifier: neither camera can measure hardness

Determinative mineralogy runs on tests, not looks. All four take seconds and none survives a photograph.

Streak. Rub the specimen on the unglazed back of a bathroom tile. The powder colour is nearly constant within a species, which body colour is not: hematite specimens are silver, red or black and all three streak rusty red-brown. It also separates gold from pyrite instantly — gold streaks gold, pyrite streaks greenish-black.

Hardness. Fingernail 2.5, copper coin 3.5, steel knife or window glass 5.5, quartz 7. Scratch a fresh face and rub the mark with a thumb first — steel drawn across quartz leaves a grey smear that looks like a scratch and wipes off, meaning the mineral is harder than the steel.

Cleavage. Perfect cubes mean galena or halite; rhombs mean calcite; elastic sheets mean mica; curved shell-like fracture with no flat faces means quartz.

Heft. Barite is the classic case — a colourless non-metallic mineral that is obviously too heavy, which is essentially its identification.

Add one drop of vinegar, which separates the entire carbonate group in five seconds. Our free mineral key filters on exactly these, and the Mohs tool tells you what a scratch result means.

A photograph gives a camera colour, form and surface texture. Colour is the least reliable property a mineral has, and this is not a marginal effect.

Quartz occurs colourless, purple, pink, smoky, yellow and black — six varieties with six names and one mineral. Fluorite comes in every colour there is. Corundum is a ruby when red and a sapphire when blue and is the same species. Meanwhile completely unrelated minerals converge: calcite and quartz are both clear and glassy, and one is soft enough for a coin and fizzes in acid.

Any image-based tool is working primarily from the one character that varies most within species and least between them. That is the structural reason both apps on this page have a ceiling, and it applies equally to ours.

Rock Identifier is a competent dedicated product with 79,000 ratings at 4.7, and its collection tracker is a genuine feature for somebody building a cabinet. On tumbled stones and crystals — which is what most people are actually holding — it performs reasonably, because those are polished, distinctive and photographed well.

Orvik is broader rather than deeper. The argument is that a beach walk produces a stone, a shell, a feather and a plant, and carrying one app for all of them is worth something. We also show ranked near-misses rather than one confident name, which matters more in mineralogy than almost anywhere else because the look-alikes are so close.

Neither replaces the tests, and any claim otherwise from either of us would be false.

Apps in this category attach estimated values, and those numbers deserve scepticism on their own terms.

Gem value depends on weight, colour grade, clarity grade, cut quality, treatment history and origin. A photograph gives approximate colour. Treatment alone decides much of it and is invisible in an image: essentially all blue topaz is irradiated, nearly all tanzanite heated, most emerald oil-filled.

And the simulants are photographically indistinguishable. Cubic zirconia and moissanite look like diamond in any image, and the popular "it scratches glass" test proves nothing because quartz scratches glass too. What separates them is thermal conductivity and double refraction. Our free gemstone key lists the simulants beside each real stone for that reason.

Everything the free keys ask for costs less than one year of either app.

• An unglazed white tile — the back of a bathroom tile or the foot ring of a mug. Free.
• A copper coin and a steel knife. The two hardness points that matter most.
• White vinegar in a dropper. Separates the carbonates instantly.
• A 10× hand lens. The best-value purchase in geology.
• A small magnet on a thread, which shows weak attraction a held magnet hides.

Then break the specimen. Every description in every key refers to a fresh face, because weathering softens and discolours the outside of nearly everything — and a photograph of a weathered surface is a photograph of the weather.

Prices, tiers, ratings, versions and update dates are read from each app's US App Store listing on 2026-09-03 through Apple's Lookup API and product pages, ours included. Nothing is typed from memory and the build fails if a price appears in the prose.

This page argues that a camera cannot finish the job, which is true and is not the same as saying it is useless. Three cases where photography earns its place.

Structures rather than minerals. A geode, a banded agate, a fossil in matrix, a vesicular basalt full of gas holes, a conglomerate of rounded pebbles — all identified by pattern at a scale a photograph captures well, and where a hardness test tells you nothing.

Narrowing before you test. A first guess from an image tells you which two properties to check, which is faster than working a key blind.

Recording a specimen you cannot take. An outcrop, a boulder, a museum case. A photograph with a scale object and a note of the location is something you can work on later — and location narrows candidates as hard as any test, because checking the bedrock unit under where you stood eliminates most of a key before you start.