Are Crystals Rocks?
1. Crystals are Not Rocks
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 Pictures
Crystals are Not Rocks
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 here.
- Are crystals rocks?
- What 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 used primarily by people who use these materials for their metaphysical healing properties.
'Crystals' is not used or even recognised by those whose interest in these objects is purely scientific.
Rocks and minerals have been used for thousands of years for their healing properties. The word 'crystals' was popularised by the New Age movement of the 1970s and 80s. It's now widely used in what has become a multi-billion dollar industry. It even includes man-made materials like opalite and goldstone.
In the vast majority of cases the word is a misnomer and from a geological perspective, is incorrect.
The following two photos are crystals in the geological sense of the word. The first is quartz and the second is 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.
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 crystal structures. The external shape of the crystal is defined by the internal arrangement of atoms or the pattern of 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 made 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 graphic is an example of a highly ordered repeating arrangement of atoms.
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.
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 structure 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 used for flavouring food. The cube-shaped crystals which are 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 or an amorphous solid.
Opal, shungite, jet (type of coal), moldavite, pearl and amber (fossilised tree resin) are a few 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 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 can contain traces of other minerals or substances. Impurities often cause a change in colour or introduce various patterns. Fluorite in its purest form is colourless but impurities cause it to change colour.
The deep red of ruby is caused by impurities of chromium in the mineral corundum. When impurities of iron and titanium are present corundum turns blue. Blue corundum is sapphire.
What Exactly are Rocks?
Rocks are a combination of minerals that 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 becomes 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. Molten rock is a mixture of minerals and dissolved gases. It remains fluid because of immense heat and pressure.
Rocks which formed as magma cooled and solidified beneath Earth's surface are known as intrusive igneous rocks. Those which formed from lava from the vent of a volcano are extrusive rocks.
Both 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 be very large.
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 non existent.
During the cooling process minerals in the molten rock crystallise but the rock is still made up of minerals not crystals.
To be classified as a mineral a material must be crystalline meaning it's made up of crystals.
The third main rock type is metamorphic. This material is formed from existing rocks that have undergone a substantial change 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. The different colours in marble are the result of impurities introduced during this process.
Lapis lazuli is often referred to as a mineral but it's a metamorphic rock. Made up of various 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 plus others.
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 in the literal sense of the word.
'Crystals' is used almost exclusively by those who use rocks and minerals for their metaphysical healing properties.
Whilst the rarity of these materials varies widely, it should be remembered 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 gypsum is crystalline. Tableware such as cups and plates are produced from crystalline materials. You may be surprised to learn 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 first picture in our article is a cluster of quartz crystals (variety amethyst). The mineral in the second picture is fluorite from New Mexico. Obsidian is the mineraloid in the third picture. The final photo is lapis lazuli included with calcite and pyrite.
The first three pictures are clickable and redirect to the original non-compressed photo. Image 1 and 2 courtesy of Stan Celestian.