What Is a Mineral? The 5 Criteria Explained

A mineral is not just any solid stuff in the ground — it has to pass five specific tests. Learn the tests and you can sort almost anything into mineral or not-a-mineral.

Uni Year 1Earth science
⏱️ About 16 min
What Is a Mineral? The 5 Criteria Explained — illustration
Illustrative image (AI-generated).

Hold a diamond, a lump of coal, a piece of obsidian glass, and a seashell. They are all solid, natural, and come from the Earth — yet only the diamond is unambiguously a mineral. Why? Because geologists have agreed on five precise tests a substance must pass, and most solids fail at least one. Master the five criteria and the word mineral stops being vague.

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The big idea: A mineral is a naturally occurring, solid, crystalline substance with a definite (but not always fixed) chemical composition, generally formed by non-biological processes. Those five ideas are a checklist: miss any one and the substance is something else — a rock, a glass, an organic material, or a synthetic.
🎯 By the end, you'll be able to
  • State the five criteria that define a mineral and explain why each matters
  • Apply the five criteria to decide whether a given substance (coal, obsidian, diamond, lab-grown crystal) is a mineral
  • Explain why a definite chemical composition permits a range, not just one fixed formula (solid solution), using olivine as an example
  • Describe how modern mineralogy treats the inorganic criterion for biogenic crystalline materials such as shell calcite
📎 Helpful to know first

A five-part test, not a feeling

People use the word mineral loosely — for vitamins, for water, for anything dug out of the ground. Geologists use it precisely. A mineral is a substance that satisfies all five of these criteria at once:

  1. Naturally occurring — formed by geologic processes, not manufactured.
  2. Solid at ordinary surface temperatures.
  3. Crystalline — its atoms sit in an orderly, repeating internal arrangement.
  4. Definite chemical composition — expressible by a chemical formula (with some allowed variation).
  5. Generally inorganic — not produced primarily by living organisms as the key feature (see the nuance below).

A substance must pass all five. Fail one, and it earns a different label.

🔑 Mineral = naturally occurring + solid + crystalline + definite composition + (generally) inorganic
Memorise the five as a single checklist. The two criteria that trip people up most are crystalline (a glass like obsidian is solid but its atoms are jumbled, so it is not a mineral) and definite composition (which still allows a range, as we will see). Rocks, by contrast, are aggregates of one or more minerals — minerals are the building blocks, rocks are the finished masonry.
Common substances sorted into mineral and not-a-mineral by the five criteria Mineral — meets all five criteria Not a mineral — which criterion fails Diamond (C) natural, solid, crystalline, definite C Quartz (SiO₂) the most common crust mineral Halite (NaCl) rock salt; grows cubic crystals Pyrite (FeS₂) fools gold; a sulfide Calcite (CaCO₃) the mineral that makes limestone Obsidian volcanic GLASS — atoms not ordered (not crystalline) Coal from plant matter — ORGANIC, no fixed formula Pearl BIOGENIC; not a single crystalline lattice Liquid water not a SOLID (its solid, ice, is a mineral) Lab-grown diamond not NATURALLY occurring — a synthetic Is it a mineral? Apply the five criteria Each failure is about one specific criterion — not about being rare, pretty, or valuable.
Apply the five criteria and most solids get sorted decisively. Notice that the failures are each about one specific criterion — not about rarity or value.

Two criteria that deserve a closer look

Naturally occurring rules out the crystals grown in a laboratory or factory. A gem lab can grow a diamond chemically and structurally identical to a mined one; geologists call that a synthetic mineral or simply a synthetic, to keep the distinction honest. (A few older textbooks accept synthetics as minerals if they match a natural species; the cleaner rule for this course is: minerals are made by Earth, not by people.)

Solid means just that, at ordinary surface conditions. Liquid water is not a mineral — but ice, its solid form, is (it is crystalline and natural). Liquid mercury, oddly, is therefore not a mineral either, even though it occurs naturally in ore deposits.

⚠️ Misconception: all minerals are inorganic
It is commonly taught that minerals must be inorganic, full stop. The real picture is subtler. Many organisms build crystalline materials — the calcite in a clamshell, the apatite in your tooth enamel — and these biogenic crystals are chemically and structurally identical to the same minerals formed geologically. Most modern usage therefore emphasises orderly crystalline structure and a definite composition as the decisive features, and the reference list of mineral species maintained by the International Mineralogical Association (IMA) includes many such materials. The honest version of the rule is generally inorganic: most minerals form without biology, but a crystalline, definite-composition solid is treated as a mineral whether or not life was involved in making it.

Definite composition does not mean one fixed formula

The fourth criterion says a mineral has a definite chemical composition — but definite does not mean a single rigid formula. Many minerals allow solid solution, a range in which two similar ions substitute for each other in the same crystal site.

The gemstone olivine is the classic example. Its formula is written (Mg, Fe)₂SiO₄: magnesium and iron are close enough in size to swap in and out of the same spots, so one olivine crystal can be magnesium-rich, another iron-rich, and anything in between. They are all still olivine — same structure, same name, a range of allowed compositions. That is what definite-but-variable means.

✨ A formula alone does not name the mineral
Here is the surprise that sets up the next lesson: two different minerals can share the exact same chemical formula. Diamond and graphite are both pure carbon (C). Calcite and aragonite are both CaCO₃. Same recipe, different mineral — because their atoms are arranged differently. Structure, not just chemistry, defines a mineral. This idea (polymorphism) is the whole point of the Crystal Structure lesson.
📝 Worked example: Decide whether each of these is a mineral, and if not, name the failed criterion: (a) obsidian (volcanic glass), (b) pearl, (c) a diamond grown in a laboratory.
  1. (a) Obsidian — NOT a mineral. It is natural and solid, but it cooled so fast that its atoms froze in a disordered jumble. It is a glass, lacking an orderly crystalline structure. Fails: crystalline.
  2. (b) Pearl — NOT a mineral (in the strict sense). It is solid and made of crystalline aragonite, but it is built by a living organism (a mollusc) as an organic composite with the protein conchiolin, so it has no single definite composition or lattice. Fails: definite composition / a single crystalline structure.
  3. (c) Lab-grown diamond — NOT a mineral under the strict rule. It is solid and perfectly crystalline, but it was manufactured, not formed by geologic processes. It is a synthetic. Fails: naturally occurring.
✓ Obsidian fails crystalline; pearl fails definite composition; lab diamond fails naturally occurring. None of the three is a mineral under the strict five-criteria rule.

Check your understanding

1. Which of these is the best one-line definition of a mineral?
A mineral must be naturally occurring, solid, crystalline, and have a definite composition (and is generally inorganic). A glass fails crystalline; a lab crystal fails naturally occurring.
2. Obsidian is a natural, solid Earth material. Why is it not a mineral?
Obsidian is volcanic glass — its atoms are frozen in a disordered jumble, so it fails the crystalline criterion even though it is natural and solid.
3. Olivine is written (Mg, Fe)₂SiO₄, with magnesium and iron substituting for each other. How does this fit the definite-composition rule?
Definite does not mean one rigid formula. Mg and Fe are similar ions that share a crystal site, so olivine spans a composition range while remaining one mineral species.
4. Diamond and graphite are both pure carbon. Why are they considered different minerals?
Same formula (C), different atomic arrangement (structure). Because structure defines a mineral, diamond and graphite are distinct species — a relationship called polymorphism.
🔎 Related lessons
Next: Crystals & Crystal Lattices The Silicates: Earth's Most Common Minerals From Atoms to Minerals to Rocks (M1)
✅ Key takeaways
  • A mineral is a naturally occurring, solid, crystalline substance with a definite (but not fixed) chemical composition, generally inorganic.
  • All five criteria must be met; failing one yields a different label (glass, rock, organic material, or synthetic).
  • Obsidian fails crystalline (it is a glass); coal fails definite composition (organic); lab-grown diamond fails naturally occurring (it is synthetic); ice passes all five.
  • Definite composition allows solid solution — e.g. olivine (Mg,Fe)₂SiO₄ — so one mineral can span a composition range.
  • Modern usage emphasises crystalline structure and definite composition; biogenic crystals like shell calcite are generally accepted as minerals.
➡️ We keep saying crystalline structure decides whether something is a mineral — and even which mineral it is. So what exactly is a crystal lattice, and why does the same formula make diamond or graphite depending on the arrangement? That is the next lesson.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of minerals.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.