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Are Crystals Rocks?

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Contents

1. Difference Between Rocks and Crystals
2. If Crystals are Not Rocks...
3. Crystals Made Up of Atoms
4. Is a Mineral a Rock?
5. What are Rocks?
6. The Word 'Crystals'
7. Article Photos

The Difference Between Rocks and Crystals

When it comes to understanding the difference between crystals rocks and minerals there's so much confusion.  These three naturally occurring solids are all different from each other and hopefully this article will make it easier to understand what each one is.  

If you're looking for the answer to one of these questions, you'll find it right here.

  • Are crystals rocks?
  • What exactly are minerals?
  • What are rocks?

If you don't want to read the entire article and just want a quick answer, then here it is;

Crystals are made up of atoms so they're not rocks.  Minerals are made up of crystals so they're not rocks either.  Rocks are made up of several different minerals which is why they're rocks not minerals.

For a more detailed explanation, read on. 

If Crystals are Not Rocks What are They?

In recent years the word 'crystals' has become a generic term for rocks and minerals.  It's not used or recognised by anyone whose interest in these naturally occurring solids is scientific.  The word tends to be used by those whose interest in rocks and minerals is metaphysical.

You won't hear a geologist or mineral collector who does not use rocks and minerals for their 'healing properties' refer to them as 'crystals'.  Unless of course they're talking about an actual crystal in the true sense of the word.

Stones have been used for thousands of years for their metaphysical properties.  The word 'crystals' was popularised by the New Age movement of the 1970's and 80's.  It's now widely used in what has become a multi-billion dollar industry.  It even includes man-made substances such as opalite and goldstone.

Sadly in the vast majority of cases the word is a misnomer and from a geological perspective, is incorrect.

The following two photos are of crystals in the geological sense of the word.  The first features terminated quartz crystals, the second cube-shaped fluorite crystals.  Both pieces would still be referred to as minerals.  A mineral is the object as a whole and although crystals are visible, in other minerals they may not be.

To really understand the difference between crystals rocks and minerals it's necessary to talk about each one individually. cluster of mostly colourless quartz crystals

 blue fluorite cube shaped crystals

Crystals Made Up of Atoms

For a naturally occurring solid to be described as a crystal it must have a highly ordered arrangement of atoms that repeat in a three dimensional pattern.

It takes billions of atoms to form one single crystal.  The way the atoms come together is known as the crystal structure.

There are seven different crystal structures. The external shape of the crystal is defined by the internal arrangement of atoms or 'the crystal structure'.

In the photos above you can see crystals with two completely different shapes. This is because of the internal arrangement of atoms.  All crystals exhibit symmetry because they're built up of repeating geometric (regular lines and shapes) patterns.  

To put the size of an atom into perspective, more than one million would fit easily onto a pinhead.

The following image is an example of a highly ordered repeating arrangement of atoms.

Example of a highly ordered arrangement of atoms in a crystal

The final form a crystal takes is known as its habit.  Cube-shaped crystals also known as isometric are one of the most common and simplest shapes found in crystals and minerals.

Salt is made up of isometric crystals, a snowflake is made up of hexagonal or a six sided crystal.

Crystals often form when liquids cool and harden.  The process is called crystallisation.  As the arrangement of atoms grow the crystal structure develops.  The mass then becomes a crystalline solid.  Crystalline describes a naturally occurring solid made up of crystals.

Crystals are not rocks because they're made up of individual units of atoms all of which are identical.  These structures are called unit cells.  A single crystal can be made up of just a few or billions of unit cells.

The cell is reproduced over and over again in all directions.  This is the internal structure of the crystal and is known as the crystal lattice.  The shape of the unit cell and symmetry of the lattice determines the external shape of the crystal faces.

All states of matter which includes almost everything in the universe is made up of atoms.  The three states of matter are gases, liquids and solids.

For a solid to be crystalline the atoms must come together in a highly ordered repeating pattern.  Where this doesn't happen, the substance is non-crystalline or void of crystals.  The term mineraloid may then be used instead.

Is a Mineral a Rock?

A mineral is a naturally occurring, inorganic solid with a crystalline structure (made up of crystals).  Inorganic means it does not come from or has been formed by any living matter.

Quartz and fluorite are minerals but pearl is not.  That's because it's produced by marine oysters and freshwater mussels which are organisms.  An organism is an animal, plant or single-celled life form.

Wood is not a mineral because it's organic but petrified wood (aka fossilised wood) is.  This is because the organic matter in the original organism has been replaced with quartz, calcite, pyrite or occasionally opal.  With the exception of opal these materials are crystalline meaning they're minerals.

Minerals form when molten rock (magma or lava) cools above or below Earth's surface.  They can also form by separating from water rich in dissolved minerals.  Using halite as an example, this mineral forms as saltwater evaporates.  The dissolved minerals then crystallise to form a substance which we use for flavouring food.  The cube-shaped crystals which are relatively soft and dissolve quickly are better known as salt.

Naturally occurring solids described as non-crystalline are void of any significant crystal structure.  This is because the atoms do not come together in a highly ordered repeating pattern.  These substances are known as mineraloids.

Opal, shungite, jet (type of coal), moldavite, pearl and amber (fossilised tree resin) are some examples of mineraloids.  Another is obsidian which is a volcanic glass.  It forms as lava cools on the surface of the earth or beneath the ocean.  The molten rock cools so rapidly that crystals do not have time to grow.

A mineral is not a rock because rocks are made up of several different minerals.

There are approximately 5,000 thousand minerals on Earth yet the vast majority of rocks are formed from a combination of some of the most common.  Known as rock-forming minerals, they include feldspars, quartz, amphiboles, micas, olivine, garnet, calcite and pyroxenes.

Minerals are naturally pure but often contain impurities of other minerals or substances.  The impurities may cause a change in colour or introduce various patterns.  Fluorite in its purest form is colourless but impurities bring about a change in colour.

The deep red colour of ruby is caused by impurities of chromium in the mineral corundum.  When impurities of iron and titanium are present it turns blue.  Blue corundum is sapphire. 

rough obsidian which is volcanic glass

What Exactly are Rocks?

Rocks are a combination of different minerals which come together through various geological processes.  The specific chemical composition determines the type of rock that forms.

Unlike a mineral a rock can be organic meaning it's made up of materials that were once part of a living organism.  Only sedimentary rocks can be organic.  An example is coal which formed over millions of years from compressed plants.

Sedimentary rocks often contain shells and skeletons of marine organisms whose remains accumulated as sediment.  Over time the sediment becomes buried, compaction squeezes out any water and cementation gradually leads to layers bonding together.  The hardened mass then slowly transforms into sedimentary rock.

The three main rock types are sedimentary, igneous and metamorphic.  Sedimentary rocks form from sediments and particles which can be organic or inorganic.

Igneous rocks form from molten rock that cools and solidifies either above or below Earth's surface.  It's known as magma or lava depending on whether it's above or below the surface.  Molten rock is a mixture of minerals and dissolved gases.  It remains in its fluid state because of immense heat and pressure within Earth's crust.

Rocks which formed as magma cooled and solidified beneath Earth's surface are known as intrusive igneous rocks.  Those which formed from lava are extrusive rocks.

Both rock types contain crystals whose size is determined by the length of time it took for the magma or lava to cool and solidify.  Where this happens over thousands to millions of years crystals have plenty of time to grow so will often be very large.  An example is granite

When lava cools and solidifies rapidly on the surface or beneath the ocean there's less time for crystal growth.  Crystals will therefore either be very small or in the case of obsidian, almost non existent.

Rocks form from molten magma or lava that has solidified and both contain minerals.  During the cooling process it's the minerals that crystallise so strictly speaking, the rock is made up of minerals not crystals.

To be correctly classified as a mineral a material must be crystalline.  To be crystalline it must have a highly ordered repeating arrangement of atoms.

The third main rock type is metamorphic.  This material is formed from existing rocks that have undergone a substantial change usually caused by intense heat and pressure.  The process which mostly takes place beneath the surface is known as metamorphism.  The word comes from Latin for 'changed form'.

Marble is a metamorphic rock that was originally limestone.  Having metamorphosed when the calcite within the limestone recrystallised, it formed a much denser material with similar sized crystals.  The different colours in marble are the result of impurities introduced during the process of metamorphism.

Lapis lazuli is often referred to as a mineral but it's metamorphic rock.  Made up of several different minerals, it occurs in limestone which has been metamorphosed.  The result is the formation of marble.

The rich blue colour comes from the mineral lazurite but other minerals present can include pyrite, calcite, sodalite, diopside, amphibole, feldspar, mica and others.    

rough lapis lazuli

The Word 'Crystals'

When the word 'crystals' is used to describe rock and mineral-like solids it's not being used literally.  Its use describes a whole host of materials most of which are visually attractive to the general population.  They may or may not be crystalline and the word is used mostly for materials used for metaphysical purposes.

Whilst the rarity of these materials varies widely, it should be remembered that crystals in the true sense of the word are extremely common.  They're present in ice which is crystallised water, in salt, in sugar which is crystallised sucrose and sand which is crystals of silicon dioxide.

Granite often used for kitchen worktops is made up of crystalline minerals.  The plaster on our walls which is made from gypsum is crystalline.  Tableware such as cups and plates are produced from crystalline materials.  You may be surprised to learn that lead crystal is not crystalline.  Despite the name, this glass is an amorphous solid which is another way of describing an object void of any significant crystal structure.

The word 'quartz' when used in reference to kitchen countertops is another misnomer.  It was created for the purpose of marketing because the industry wanted a cheaper alternative to granite.  Unlike granite, quartz countertops are man-made.  Although they contain some quartz it's only a small percentage.  Several different types of stone is used in quartz worktops.  They have all been ground down and mixed together with resins.  A binding agent is then added so the end product has the look and feel of natural stone.

Article Photos

Our first photo is a cluster of quartz crystals. The variety is amethyst. The second mineral is fluorite from New Mexico.  Obsidian is the mineraloid in our third photo.  Our last photo is lapis lazuli with bands of calcite and pyrite.

The first three photos are clickable and redirect to the original non-compressed photo.  Image 1 and 2 courtesy of Stan Celestian. 

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