Rocks & Minerals

There are 20 articles here , published between January 2026 and June 2026, and they answer questions rather than catalogue species: how to tell two similar things apart, what a particular mark or shape means, what to do once you know. They are the long form.

The faster route, when you have the thing in front of you, is one of the 4 free keys for this section — 122 entries between them. A key does not ask you to describe what you are looking at in words; it asks a series of questions you can answer by looking, and the list shrinks as you go. Nothing is hidden behind the filter: every entry is written into the page, so the whole catalogue is readable with scripting switched off, and each one carries the single sentence that separates it from its nearest look-alike.

There is a third thing on the site worth knowing about from here, because it answers a question this section cannot. People arriving at an identification problem usually end up comparing apps, and the honest answer to "which one" depends on facts that change — what each costs this month, what it locks behind a subscription, who publishes it. Those pages exist under compare and alternatives, and not one figure on them is typed by hand: every price and every in-app purchase list is read from a dated App Store snapshot, and the build fails if a currency amount ever appears in the editorial text. It is the same discipline as the numbers below, applied to a subject that goes stale faster.

Reading a shortlist without being fooled by it

Whatever brings you here, the failure mode is the same and it is worth naming. A key, an app and a search all hand you a shortlist, and a shortlist feels like an answer. It is not: it is a set of candidates that survived the characters you happened to check, and the one at the top is usually the one that is common rather than the one that is right. That is often the same thing, which is exactly why it is dangerous when it is not.

Three habits fix most of it. Check a character you have not used yet — if the candidate survives a test drawn from a different part of the organism, the identification is much stronger than one built on colour alone. Check where you are: an enormous share of confident misidentifications are species that do not occur within a thousand miles of the observer, and geography alone settles them. And notice when the answer does not matter — a great many identifications are curiosity, where being wrong costs nothing, and the small number where it does cost something are worth treating completely differently.

Where the consequence of an error is medical — anything eaten, anything that bit somebody, anything a child had in their hands — no page here and no app is the right tool. Describe what you have to somebody qualified, and keep the specimen or a photograph. The keys are built to make that description accurate, which is a genuinely useful thing to be, and it is a different job from telling you it is safe.

The mineral and gemstone keys in this section add 48 entries with a published chemical formula, Mohs hardness range and crystal system, taken from Macrostrat. Hardness is the one property here you can test yourself with a fingernail, a coin and a knife blade, which is why the keys lead with it rather than with colour.

The fossil key adds 20 groups with a first and last appearance in millions of years, from the Paleobiology Database — so the age of the rock a specimen came out of can rule most of the list out before you look at the specimen at all.

Every figure on this page comes from the same snapshot, and the whole thing is published as one downloadable file: open field data, CC BY 4.0, with each source's own licence listed. If a number here disagrees with the source database today, the source is right and this snapshot is old.

Free keys in this section: What kind of rock is this? · Which mineral is this? · What gemstone is this? · What fossil is this?