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

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

Crystals, are they Rocks or Minerals?

Are crystals, rocks and 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 these materials 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 have been, and continue to be, associated with status, wealth, and power.

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

Today, rocks and minerals, which have become known collectively as 'crystals', are widely used to promote health and well-being.  

Although crystal healing is a pseudoscience, it's hugely popular around the world. In recent years, this alternative therapy has exploded and is now a multi-billion-dollar industry.

Although the words 'crystals', 'rocks', and 'minerals' are often used interchangeably, each one 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' describes a naturally occurring solid made up of more than one mineral. Rocks tend to be relatively hard, hence the idiom 'hard as a rock'.

The main types of rock are igneous, sedimentary, and metamorphic. All three form through a different geological process.

Minerals, which also form through a geological process, are naturally occurring solids with a specific chemical composition and crystalline structure.

Chemical composition refers to the identity and number of chemical elements in a compound. Water (the compound) is made up 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 called crystallography, rocks, petrology, minerals, 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?

A crystal, in the true geological sense of the word, 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 'not formed from living matter.'   

A typical crystal contains billions of atoms. Actually, it contains quadrillions. The shape they form when they come together is known as the crystal structure.  

'Crystal structure' refers to the ordered arrangement of atoms within a crystalline solid. When atoms come together to form a crystal, they organise in a specific pattern which defines the external geometric shape of the crystal.

Although the structure of the atoms can influence the overall shape of a crystal, factors such as growth conditions and impurities can also affect its final appearance.

Crystal formation and their subsequent growth is called crystallisation.

The crystal structure of a specific mineral is always the same. If it 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 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. Minerals are always made up 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, which are often seen in pyrite and fluorite, are one of the simplest and most common shapes.

Matter is any naturally occurring substance with a mass that 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. The material will be non-crystalline if the atoms are arranged in a disorganised or random pattern.

Obsidian, which is a natural volcanic glass, is a non-crystalline solid. Window glass, which is 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,' which means 'having no specific shape or form,' is also used. However, 'amorphous' can describe non-crystalline solids that are natural or man-made.

Crystals often grow when liquids cool and solidify. If the liquids cool too quickly, there isn't enough time for atoms to organise in an orderly, repeating pattern.  Therefore, 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. As their concentration increases, the excess can no longer remain dissolved, which causes 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, which means they're made up of two or more chemical elements. Quartz is composed of one atom of silicon and two 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 (one type of atom) 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. Approximately 5,000 minerals exist on Earth, yet most rocks are formed from a combination of some of the most common.

Granite, which is a common igneous rock, is made up of the rock-forming minerals quartz, mica, and feldspar.

Even though the minerals that make up rocks are crystalline, the rock itself is not made of crystals in the same way as minerals. Rocks are aggregates or collections of minerals, and their overall structure is more complex than that of an individual crystal.

In rocks, minerals are often intergrown or interlocked with one another forming a solid mass without distinct crystal boundaries.

Pressure, temperature, and the presence of other minerals influence the arrangement and orientation of the minerals within the rock. This is why rocks exhibit a range of textures and structures.

Some, particularly igneous and metamorphic rocks, can contain visible crystals. Granite is one example. This is because slow cooling or intense pressure and heat allowed larger crystals to grow over long periods.

Even where larger crystals are present, the rock as a whole is not composed entirely of crystals. It's a combination of minerals and other substances.

One way to understand a rock's nature and composition is to think of it as a solid mass made up of individual grains tightly compacted together. Each grain represents a separate mineral, and the nature of the grains and how they fit together define the rock's hardness.

Rough, angular grains that fit tightly together restrict the amount of space left for moisture, making this type of rock hard and non-porous.

Finer grains, which are typically more rounded, don't lock together as tightly. This leaves space for moisture and air, which makes the rock softer and more likely to be porous.

All rocks begin as igneous rocks but are altered over time by natural geological processes. Igneous rocks form when magma (molten rock below the surface) or lava (molten rock above the surface) cools and solidifies.

Sedimentary rocks are composed of sediment that accumulates underwater. Over time, layers of sediment build up and, under pressure, become compacted together. Through the process of cementation, the mass slowly transforms into rock.

A rock that undergoes profound transformation due to extreme heat or pressure becomes a metamorphic rock.

Minerals are only stable within a certain range of temperatures and pressures. When these conditions are exceeded, they break down and react with other minerals within the rock, forming new minerals. This process, known as metamorphism, occurs primarily deep within Earth's crust.

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, taken by Stone Mania, is clickable and redirects to the original image.

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