Explainer: What is a rock?

Everyone is familiar with rocks. It’s hard not to be. They’re everywhere – we walk on them, skip them across ponds, dig them out of our gardens, and stare appreciatively at them as they form mountains and canyons.

So, we all know what a rock actually is, right?

Turns out rocks are pretty hard to define

The definition of a rock is a bit woolly, often contradictory, and hard to pin down – even for geologists.

In the broadest sense, a rock is a collection of other things. Officially, it is a collection of one or more types of minerals, held together in a solid mass.

There are three main types of rocks, classified according to how they formed.

1. Igneous rocks are formed when molten rock (magma) cools and hardens, as a result of volcanic activity. Examples include basalt and granite.
2. Sedimentary rocks are formed when sediments are compacted and cemented together (like shale or sandstone), or minerals are formed from plant or animal remains (like limestone or coal).
3. Metamorphic rocks are formed when heat and pressure alter an existing rock, most commonly inside the Earth’s crust. Examples include slate, marble and schist.

A rock can be transformed from igneous to sedimentary to metamorphic rock at different points in time, in any given order.

But the key point is that a rock must be made up of one or more minerals.

Okay, but what exactly is a mineral?

Good question.

Minerals must:

• be a naturally occurring solid,
• be formed by geological processes
• have a specific chemical formula, and
• have a fixed lattice structure.

Common examples include quartz, feldspar, mica and clay. Graphite and diamond are both minerals – interestingly, they have the same chemical structure but different crystal lattice. Elements can also be minerals, and surprisingly ice is a mineral too, as it is inorganic, solid and crystalline. However, water is definitely not a mineral because it doesn’t have that crystal structure.

The structure is crucial. Some solids like glasses or opals have a well-defined chemical formula but aren’t arranged in a lattice, and so don’t count as minerals. But here’s where it gets tricky – even though glass and opal aren’t minerals, they could still be part of a rock.

This is because there’s a caveat to the definition of a rock: it can be made up of one or more minerals or mineraloids.

Mineraloids are a slightly fuzzier group of mineral-like substances that lack the crystal structure – like obsidian, which is an amorphous (uncrystallised) glass, or mercury, which is liquid at room temperature.

Hold on, who gets to decide what’s a mineral or not?

Official definitions are presided over by the International Mineralogical Association, which is also responsible for sanctioning newly found minerals. (Fun fact: around 100 new minerals are discovered each year. Many of them have properties that are incredibly useful in technology like superconductors; graphene is a good example of this.)

There are more than 5,000 approved minerals – but no equivalent list of approved rocks.

So given all of this, what exactly counts as a rock?

Quickfire round: Rock or not?

Coal: This is definitely a rock, even though it’s made of organic materials – ancient vegetation that has been buried and transformed by heat and pressure over millennia. Officially it’s called a biogenic sedimentary rock.

Fossils: Yes, absolutely. The process of fossilisation involves changing the original object’s composition, replacing organic materials – whether a shell or living tissue – with minerals. (There’s a famous fossil site in the UK where ammonites are found made entirely of pyrite, otherwise known as fool’s gold.)

Amber: This is technically a rock too, in the same way that coal or fossils are considered rocks. It’s a biological material (tree sap) that has undergone a geological process to change its composition, becoming harder and more crystalline. Same deal with petrified wood: it’s a fossil formed when plant material is buried and transformed.

Glaciers: Since ice is a crystalline form of water and therefore a mineral, a glacier is in fact a rock. It’s classed as a monomineralic rock, made up of just one kind of mineral – tens of thousands of snowflakes that have been crushed under the weight of the snow above to form dense ice crystals.

Salt: Salt is a rock, whether it comes from salt pans (like cheap white table salt) or mined from deep underground (like pink Himalayan rock salt). It’s mostly composed of one mineral – sodium chloride, or NaCl – and has a defined cubic crystal structure.

Meteorites: Yep, space rocks are still rocks. They’re usually made up of different minerals than a typical Earth rock – largely silicates – and some are blends of rock and metal, like iron. Asteroids and comets are also rocks.

Magma: Despite being called molten rock, this is not actually classified as a rock because it’s not solid. It’s a liquid that lacks a crystalline structure, but as it cools a number of different minerals form, including quartz, olivine and pyroxene – and the collection of these bound together forms a rock. Different combinations of these minerals become different rocks; pyroxene and olivine become basalt, while quartz and feldspar form rhyolite and granite.

Obsidian: This is a tricky one. By some accounts, volcanic glass or obsidian is not a true rock because it doesn’t have a crystalline structure – but it’s also classed as a mineraloid, and since rocks can be made up of mineraloids, it seems that obsidian might be a type of igneous rock.

Gemstones: There’s no formal scientific definition of a gemstone – it’s simply a mineral that is attractive to humans (usually due to its colour, clarity or hardness) and therefore is precious. But most “gemstones” are minerals and therefore can comprise rocks.

Sand: Technically, sand could be classified as just a bunch of very tiny rocks. The vast majority of sand grains on a beach are single minerals so arguably they are monomineralic rocks, but calling a single grain a rock seems a bit of a stretch. Geologists seem more comfortable calling an object a rock when two or more minerals are stuck together – the “glue” cementing them and hardening the rock appears to make a difference.

So how small can a rock get?

Perhaps a better definition of a rock is something that is made up of more than one individual mineral – the minerals can have the same composition, just with different orientations or crystal lattice structures.

But a rock could be extremely small, even just a few hundred nanometres across. Cementing together clay, for example, creates shale – with individual particles so fine they would have to be viewed under an electron microscope.

How big can a rock get?

There seems to be no limit – some mountains could be called rocks. In fact, the entire Earth could even be classified as a rock. Of course, it’s not solid all the way through and the layers have different compositions, so the core would be counted as one rock, and then the mantle as another, and the crust as another.

Boy, the grey areas seem expansive

It appears to be very difficult to clearly define a rock – and different geologists will probably have different views.

The lines seem particularly blurred around objects with artificial or biological origins.

What about hard objects created by the human body, like gall stones or teeth? Tooth enamel and bone mineral are very close in composition to a mineral called apatite, which is also formed in rocks. Some geologists argue that if a substance is made of minerals and is solid at normal surface conditions, it should count as a mineral – but according to the International Mineralogical Association, biogenic objects like teeth are not minerals. However, if a tooth underwent a geological process – such as being buried, heated, and given time to form a regular crystal structure – it would then become a rock.

Since rocks are defined as naturally occurring, materials like concrete don’t count – but what if concrete is buried and forms part of a newly created rock? And what about plastic? There have been reports of a new type of geological material on Kamilo Beach in Hawaii, formed when plastic debris was heated by campfires and mingled with surrounding sediment or volcanic rock. Called “plastiglomerates”, these hybrid materials fuse the anthropogenic and geologic. Though traditional definitions exclude them from being rocks, plastiglomerates could be buried and become a part of the permanent geological record (even if the plastic part would degrade over thousands of years) – so would they become rocks at any point?

In fact, the International Mineralogical Association has recognised over 200 minerals that humans have had some hand in creating, contradicting their official definition.

As humans continue to pour artificial material into the strata, perhaps we’ll have to rewrite the definition.

Okay, last question: what’s the difference between a rock and a stone?

Zilch – they’re interchangeable, although geologists will probably look down their nose at you if you go around calling rocks “stones”.

But if they make any lofty comments, you can remind them that people who live in rock houses shouldn’t throw away their stones: they don’t even know how to properly define a rock anyway.

More explainers from Cosmos:

• What creates Earth’s magnetic field?
• The difference between mass and weight
• What is gravity?