A mineral is defined by five characteristics: it must be naturally occurring, solid, inorganic, possess a definite chemical composition, and have an ordered internal crystalline structure. These criteria, maintained by the International Mineralogical Association (IMA), sound straightforward, but every single one of them has edge cases that keep mineralogists debating. Understanding what each characteristic actually means in practice reveals why the roughly 6,000 recognized mineral species represent a surprisingly strict club.
Naturally Occurring
The first requirement is that a mineral forms through natural geological or cosmic processes rather than through human manufacturing. This distinction is why synthetic diamonds, lab-grown rubies, and industrially produced crystals are not classified as minerals even though they can be chemically and structurally identical to their natural counterparts. The natural-origin rule also applies to substances formed accidentally by human activity. Crystals that grow on the walls of old mine tunnels, for example, or phases that form in the slag of ancient smelters, sit in a gray area. The IMA’s Commission on New Minerals, Nomenclature, and Classification (CNMNC) has issued specific guidelines addressing these borderline cases, particularly regarding minerals found on burning coal dumps, where distinguishing natural self-ignition from human-caused fires becomes critical. Phases from such settings can be approved as minerals only if their natural, non-anthropogenic origin is convincingly demonstrated.1European Journal of Mineralogy. IMA-CNMNC guidelines for assessing the natural geological origin of minerals
The “naturally occurring” criterion also raises questions about ice. Water ice that forms in glaciers, permafrost, or polar caps meets all five mineral criteria and is formally classified as a mineral. The ice cubes in your freezer, however, are not, because you made them. Same substance, different classification, purely because of origin.
Solid
Minerals must be solid under normal conditions. This rules out liquid water, liquid mercury at the surface, and gases, regardless of their chemical simplicity or natural origin. The requirement seems obvious, but it matters at the margins. Mercury is a native element found in nature, yet because it is liquid at room temperature, it does not qualify as a mineral. (Frozen mercury in extremely cold environments is a different story and could theoretically qualify.) Volcanic gases, despite being inorganic and naturally occurring, fail the solidity test entirely.
The solidity criterion also interacts with how we think about amorphous substances. Glass formed from volcanic lava, called obsidian, is solid, naturally occurring, and inorganic, but it fails a different test (the crystalline structure requirement) and is therefore classified as a mineraloid rather than a true mineral. The solidity check is necessary but not sufficient on its own.
Inorganic
Minerals must be inorganic, meaning they are not produced by biological processes in the way we normally think of organic chemistry. This criterion is the one that generates the most exceptions and the most arguments. Coal, for instance, is derived from ancient plant material and is classified as a rock rather than a mineral, even though it is solid and naturally occurring. Pearls, made of aragonite crystals secreted by mollusks, are sometimes called minerals in casual conversation but are not classified as such because they are biologically produced.
The tricky part is that living organisms routinely produce minerals inside their own bodies. Your bones and teeth contain hydroxyapatite, an inorganic calcium phosphate mineral, organized within matrices of collagen and specialized proteins.2PubMed Central. Biomimetic systems for hydroxyapatite mineralization inspired by bone and enamel Hydroxyapatite itself is a recognized mineral, and when it forms in geological settings it passes all five tests without controversy. The mineral is the same whether it crystallizes in a rock or in a tooth. What the “inorganic” criterion really targets is whether the substance is organic in the chemical sense, meaning composed primarily of carbon-hydrogen bonds in biological molecules, rather than whether a living thing had anything to do with its formation. Shells made of calcite or aragonite are built by organisms, yet calcite and aragonite are firmly on the mineral list because their chemistry is inorganic.
Definite Chemical Composition
“Definite chemical composition” does not mean every specimen of a mineral has exactly the same formula down to the last atom. It means the composition can be expressed as a specific chemical formula or falls within a well-defined range. Quartz is always SiO₂. Halite is always NaCl. Those are straightforward cases. But many minerals are solid solutions, where one element can substitute for another within the crystal structure. Olivine, for instance, ranges continuously between a magnesium-rich end member and an iron-rich one, with the two swapping in and out depending on conditions. The composition varies, but it varies predictably within set bounds, and the overall structural framework stays the same.
This flexibility within limits is what separates minerals from random mixtures. A chunk of granite is a rock made of several different minerals, each with its own composition. A crystal of garnet is a single mineral even though garnets can contain varying amounts of calcium, magnesium, iron, and aluminum, because those substitutions follow strict crystallographic rules. The IMA’s procedures for defining distinct mineral species rely on which chemical component dominates specific sites in the crystal structure, a system that has generated considerable scientific debate over edge cases involving charge balance and site occupancy.3Mineralogical Magazine. On the definition of distinct mineral species: A critique of current IMA–CNMNC procedures
Ordered Internal Crystalline Structure
The fifth and final requirement is that a mineral’s atoms or molecules are arranged in a repeating, orderly three-dimensional pattern known as a crystal lattice. This internal order is what produces the flat faces, sharp edges, and geometric shapes that well-formed crystals display, although many minerals grow in conditions where they cannot develop visible crystal faces. The ordered structure is still there at the atomic level even when the external shape looks like a lumpy blob.
This criterion is why obsidian (volcanic glass) is a mineraloid and not a mineral. Its atoms cooled too quickly to organize into a repeating pattern, leaving it amorphous. Opal is another well-known mineraloid for the same reason: while it contains silica like quartz, its internal structure lacks the long-range order that would make it a true mineral. (Some forms of opal do have microcrystalline regions, which complicates matters, but the classic precious opal is amorphous.)
The crystalline structure requirement is also what makes polymorphism possible and interesting, a concept worth its own discussion.
When the Same Chemistry Produces Different Minerals
Because both chemical composition and crystal structure define a mineral, the same chemical formula can yield completely different minerals if the atoms arrange themselves in different patterns. Diamond and graphite are both pure carbon. Calcite and aragonite are both calcium carbonate (CaCO₃). These pairs are called polymorphs, and they can have wildly different physical properties despite identical chemistry. Diamond is the hardest natural substance; graphite is soft enough to use as a lubricant. Calcite and aragonite differ in hardness, density, and crystal shape.
The transformation between aragonite and calcite is found in both geological and biological settings, including in the nacre layers of seashells.4Crystal Research and Technology. Epitaxy and Topotaxy in the Aragonite‐Calcite Polymorphism Aragonite is the less stable form at surface conditions and slowly converts to calcite over geological time. This is why ancient fossils originally made of aragonite are often found as calcite: the chemistry stayed the same, but the crystal structure flipped to the more stable arrangement.
Polymorphism is one reason the five-characteristic definition requires both composition and structure. If only chemistry counted, diamond and graphite would be the same mineral. If only structure counted, very different substances sharing a crystal geometry would be lumped together. You need both criteria working together to distinguish minerals meaningfully.
Minerals You Will Never See
Most of the minerals people encounter, quartz, feldspar, mica, calcite, are surface or near-surface minerals. But the most abundant mineral on Earth is one that essentially no one has held in their hand. Bridgmanite, a magnesium silicate with an ideal composition of MgSiO₃, makes up the bulk of Earth’s lower mantle and is by volume the most common mineral in the planet.5PubMed Central. Phase relations of bridgmanite, the most abundant mineral in the Earth’s lower mantle It is stable only under the extreme pressures found hundreds of kilometers below the surface. Bring it up to normal conditions and it transforms into other structures.
Bridgmanite was only formally named as a mineral in 2014, after a natural specimen was confirmed in a shocked meteorite. Before that, scientists had synthesized it in the lab and studied it extensively, but under the IMA’s “naturally occurring” rule, it could not be classified as a mineral until a natural example was found. Even now, research on bridgmanite’s behavior under mantle conditions reveals surprising complexity: studies of point defects in the crystal structure show that oxygen vacancies are the most abundant type and that their concentration changes non-monotonically with depth, meaning the mineral’s character shifts in unexpected ways as you go deeper.6Journal of Geophysical Research: Solid Earth. Ab Initio Evaluation of Point Defects in Bridgmanite Under Lower Mantle Conditions
High-pressure minerals also form during violent events at the surface. When meteorites slam into the Moon or Earth, the shock pressures can transform common minerals into high-pressure phases. In the lunar breccia meteorite NWA 13120, shock pressures above 20 gigapascals converted all plagioclase fragments into maskelynite, a glass-like phase, while olivine underwent its own shock metamorphism.7Minerals. Petrography and Shock Metamorphism of the Lunar Breccia Meteorite NWA 13120 These shock-produced phases meet the five criteria (natural, solid, inorganic, definite composition, crystalline structure) and are legitimate minerals, even though they formed in an instant of extreme violence rather than through slow geological cooling.
Extraterrestrial Minerals and the Limits of the Definition
The five characteristics apply to minerals regardless of where they form. Meteorites, Moon rocks, and samples returned from asteroids all contain minerals classified under the same IMA framework used for earthly specimens. Several minerals have been discovered for the first time in meteorites rather than in terrestrial rocks, bridgmanite among them. Newer analytical techniques now allow researchers to identify and fingerprint minerals in extraterrestrial samples at the nanoscale, using methods like photothermal polarimetric nanoscopy to derive infrared spectra noninvasively from individual nanocrystallites embedded in solid matrices.8ACS Publications. Photothermal Polarimetric Nanoscopy: An Emerging Technique for Fingerprinting Minerals of Extraterrestrial Origin As sample-return missions from asteroids and eventually Mars deliver more material, the catalogue of recognized minerals is expected to grow.
This raises an interesting philosophical tension. The IMA’s classification system for mineral species relies on idealized chemical composition and crystal structure, traits that are time-independent. A mineral is defined by what it is right now, not how it got there. But some researchers have argued that this approach misses something important about minerals in the context of planetary evolution. They propose that an evolutionary classification scheme, one that accounts for when and how minerals appeared in the solar system’s history, would capture information that the static five-characteristic definition does not.9Proceedings of the National Academy of Sciences (PNAS). Historical natural kinds and mineralogy: Systematizing contingency in the context of necessity Under this view, two specimens of quartz that formed in completely different eras and through different planetary processes would be meaningfully different “kinds” even though they are chemically and structurally identical.
This evolutionary approach has not replaced the traditional five-characteristic definition, and it is unlikely to do so for practical classification. But it highlights that the definition we use is a human choice, not a law of nature. The five criteria are remarkably useful for telling minerals apart, but they were selected because they are measurable and consistent, not because they capture everything interesting about a mineral’s identity.
Common Misconceptions About What Counts
A few persistent misunderstandings are worth clearing up. First, “mineral” and “rock” are not interchangeable. A rock is an aggregate of one or more minerals (or mineraloids). Granite contains quartz, feldspar, and mica; each is a mineral, and the combination is a rock. Second, not everything hard and natural is a mineral. Amber is fossilized tree resin (organic, so it fails the inorganic test). Jet is a form of lignite coal (also organic). Both are used in jewelry and casually called “minerals” or “gemstones,” but neither qualifies.
Third, the fact that a substance has a crystal structure does not automatically make it a mineral if it fails another criterion. Sugar can form beautiful crystals, but it is an organic compound. Synthetic sapphire has perfect crystal structure and the same composition as natural corundum, but it is manufactured, not naturally occurring. Every criterion must be met simultaneously. Passing four out of five is not enough.
Finally, there is sometimes confusion about whether water is a mineral. Liquid water is not; it fails the solid test. But ice that forms naturally in glaciers or polar regions meets all five criteria and is a recognized mineral species. The distinction feels pedantic until you realize it matters for planetary science: much of what we study on Mars, Europa, and other bodies involves naturally occurring ice, and treating it as a mineral brings it under the same analytical framework used for silicates and oxides.
How New Minerals Get Approved
When a researcher believes they have found a new mineral, the specimen goes through a formal approval process managed by the IMA’s CNMNC. The proposal must demonstrate that the candidate meets all five defining characteristics and that it is distinct from every previously recognized species. In practice, “distinct” usually means a different dominant chemical component at one or more crystallographic sites, or a different crystal structure, compared to existing minerals. This process has approved thousands of species over the decades, but the criteria for what counts as a distinct species remain a subject of active scientific discussion. Some researchers have pointed out that the dominant-constituent rule used by the CNMNC can produce inconsistencies, particularly when charge-balance requirements in the crystal structure are not properly accounted for.3Mineralogical Magazine. On the definition of distinct mineral species: A critique of current IMA–CNMNC procedures
The approval process also has to grapple with the “naturally occurring” criterion in increasingly complicated situations. Minerals discovered on old slag heaps, in mine tailings, or in the corrosion products of ancient artifacts all require careful documentation that the phase formed through genuinely natural processes rather than as a byproduct of human activity.1European Journal of Mineralogy. IMA-CNMNC guidelines for assessing the natural geological origin of minerals The line between “nature did this” and “humans set up the conditions for nature to do this” is not always sharp, and the CNMNC’s guidelines try to draw it as clearly as possible while acknowledging that borderline cases will keep appearing.
Roughly two dozen to several dozen new mineral species are approved each year. Many come from unusual geological environments like fumaroles, meteorites, or deep-sea hydrothermal vents, places where extreme conditions create chemistry not found in ordinary surface rocks. Each new approval adds to the catalogue, but it also tests the five-characteristic framework against another set of edge cases, keeping the definition perpetually alive as a working scientific tool rather than a settled textbook fact.