Crystals, Rocks, Minerals: What's the Difference?

Contents 1. Crystals, Rocks or Minerals? 2. What are Crystals and Minerals 3. What are Rocks?

Crystals, Rocks and Minerals: Key Differences

Are crystals, rocks or minerals, or are rocks and minerals crystals? This question causes so much confusion.

The crystals, rocks and minerals that we enjoy today are the result of a geological process that began with the formation of Earth some 4.5 billion years ago.

Samples from three geographic regions confirm that rocks and minerals have been used for personal adornment for at least 100,000 years. Tests from one of the earliest samples showed that beads created from mollusc shells are likely to have been strung to enable them to be worn.

Rocks used for art, specifically in Africa, have been dated back 30,000 years.
 
Throughout history, crystals, rocks and minerals have been a source of intrigue and fascination. Beyond their use in art and jewellery, they have been used for tools, carried as talismans, and carved into luxury objects. All three of these naturally occurring solids have been, and continue to be, associated with status, wealth, and power.

The belief that they hold mystical powers and healing properties is well-documented. Stories and myths associated with different stones have been passed down from generation to generation.

Today, rocks and minerals used for their metaphysical healing properties have become known collectively as 'crystals.'  

Although crystal healing is a pseudoscience, it has exploded into a multi-billion-dollar industry.

Although the words 'crystals', 'rocks', and 'minerals' are often used interchangeably, each represents a slightly different, naturally occurring solid. Using the word 'crystals' as a blanket term for all three has created significant confusion.

The word rock refers to a naturally occurring solid, often composed of more than one mineral. Rocks are usually quite hard, hence the saying, hard as a rock.

The three main rock types are igneous, sedimentary, and metamorphic. Each forms through a different geological process.

Minerals are also naturally occurring solids, but unlike rocks, have a specific chemical composition and a defined crystalline structure.

Chemical composition refers to the identity and number of chemical elements in a compound. Water (the compound) is composed of two chemical elements: hydrogen and oxygen.

A substance with an internal crystalline structure is made up of crystals. Each crystal is formed from a highly ordered, repeating arrangement of atoms.

The study of crystals is crystallography, rocks is petrology, minerals is mineralogy, and gemstones, gemology. I've discussed gemstones in a separate article, which you can read here. 

These subjects fall under the umbrella of geology, which is the study of the Earth, its history, the rocks of which it's constructed, their structure, where they came from, how they have changed over time and how they continue to change.

What are Crystals and Minerals?

In geological terms, a crystal is a naturally occurring, inorganic solid composed of a highly ordered arrangement of atoms. The atoms form a repeating, three-dimensional pattern known as a crystal lattice. (Inorganic means it wasn't formed from living matter.)   

A single crystal contains not just billions but quadrillions of atoms. The way they arrange themselves when they come together is called the crystal structure.  

'Crystal structure' refers to the ordered arrangement of atoms within a crystalline solid. This internal pattern determines the external geometric shape of the crystal.

Although the atomic structure influences a crystal’s overall shape, other factors, such as growth conditions and impurities, can also affect its final appearance.

The process of crystal formation and growth is known as crystallisation.

Each mineral has a specific crystal structure. If that structure changes, a different mineral will form.

Pyrite and marcasite have an identical chemical composition. Both are iron sulphide minerals (a chemical compound of iron and sulphur). The mineral that forms depends on how it crystallises, or in other words, its crystal structure.

Another example is calcite, which has more crystal structures than any other mineral. Although its chemical composition is identical to aragonite, they're different minerals.

A mineral's crystal structure is a useful tool for identification because it's unique and specific to each mineral variety.

The crystal structure of a quartz crystal will always be the same. Made up of one atom of silicon and two of oxygen, its chemical formula is SiO₂. If it contains any other chemical elements, it wouldn't be quartz.

This diagram is an example of a repeating arrangement of atoms in a crystalline solid.

Where crystals are present in a naturally occurring solid, the material is classed as a mineral. All minerals are composed of crystals.

Some crystals can be difficult to see because of their size or because they're tightly interlocked or fused together. Others are large enough to be seen with the naked eye.

Crystals vary in size from a few millimetres to several metres. Their crystalline structure defines their external shape. Cube-shaped crystals, often seen in pyrite and fluorite, are one of the simplest and most common shapes.

Matter is any naturally occurring substance that has mass and occupies space. The three states of matter are solids, liquids, and gases. All three are made up of atoms, but the specific arrangement differs for each.

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For a solid to be crystalline, it must have a highly ordered, repeating arrangement of atoms. If the atoms are arranged in a disorganised or random pattern, the material is non-crystalline.

Obsidian, a natural volcanic glass, is a non-crystalline solid. Window glass, produced by heating sand, is also non-crystalline. The most common mineral in sand is quartz, which is a crystalline form of silicon dioxide.

Naturally occurring non-crystalline solids are correctly known as mineraloids. The word 'amorphous,' meaning 'having no specific shape or form,' is also used. However, amorphous can refer to non-crystalline solids that are natural or man-made.

Crystals often grow when liquids cool and solidify. If the cooling happens too quickly, there isn’t enough time for the atoms to arrange themselves in an orderly, repeating pattern. As a result, the solid that forms will be non-crystalline.
 
Crystals can also form through the precipitation of minerals from water. Water can only hold a limited amount of dissolved minerals and salts. When their concentration becomes to high, the excess can no longer stay dissolved, causing the particles to come together and form a solid.

An example of a mineral that forms in this way is halite, also known as rock salt.

While some crystals grow quickly, others can take thousands to millions of years. The slower the cooling process, the larger the crystals. 

Impurities within a crystal can be partly or fully responsible for changes in its colour. Heat can also enhance or change the colour of a rock or mineral by altering its chemical composition.

Minerals are chemical compounds, meaning they're made up of two or more chemical elements. Quartz is composed of one atom of silicon and two atoms of oxygen. Water is composed of two atoms of hydrogen and one of oxygen.

A chemical element contains just one type of atom. Minerals made up of a single element are known as native elements. Copper, carbon, gold, titanium, and silver are all native elements.

What are Rocks?

Rocks are usually composed of more than one mineral. Although around 5,000 minerals exist on Earth, most rocks form from some of the most common ones.

Although the minerals in rocks are crystalline, the rock itself isn’t a single crystal. Rocks are aggregates, which are collections of minerals with a more complex structure.

In many rocks, the minerals are intergrown or interlocked, forming a solid mass without distinct crystal boundaries. Pressure, temperature, and other minerals influence the arrangement of the minerals, leading to different textures and structures.

Some rocks, particularly igneous and metamorphic, contain visible crystals. Granite is a good example, where slow cooling or intense pressure and heat allowed larger crystals to grow over time.

Even when larger crystals are present, rocks aren’t made entirely of crystals. They’re solid masses composed of minerals and other substances.

A helpful way to visualise this is to think of a rock as a tightly packed mass of mineral grains. Each grain is a separate mineral, and the size, shape, and how they fit together determine the rock’s hardness and porosity.

Tightly packed, angular grains create hard, non-porous rocks by leaving little space for moisture. Finer, rounded grains leave gaps for air and water, making the rock softer and more porous.

All rocks begin as igneous, formed when molten rock, which is magma below the surface or lava above, cools and solidifies.

Sedimentary rocks form from layers of sediment that accumulate underwater. Over time, pressure compacts the layers, and minerals bind them together in a process called cementation.

When exposed to intense heat or pressure, rocks undergo metamorphism, often deep within the Earth’s crust. This transformation can create entirely new rock types and minerals as existing ones break down and new ones form in response to the changing conditions.

Article Photos

Pop-up photos: Pyrite, marcasite, calcite, gold, silver: Courtesy of Stan Celestian. Sulphur: Courtesy of Géry Parent.

The native copper is housed in the Harvard Museum of Natural History, Massachusetts, USA. The photo, by Stone Mania, is clickable and redirects to the original image.

The Lemurian quartz crystal, black obsidian, and mica are from our collection.