Are Crystals Rocks?
1. Crystals are Not Rocks
2. If Crystals are Not Rocks...
3. Crystals Are Made Up of Atoms
4. Is a Mineral a Rock?
5. What are Rocks?
6. Metamorphic Rocks
7. The Word 'Crystals'
8. 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 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 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 a combination of different minerals.
For a more detailed explanation, read on.
If Crystals are Not Rocks What are They?
In the following two photos you can see "crystals" in the geological sense of the word. The mineral in the first photo is quartz, the second is fluorite.
Although the crystals are the most obvious feature, these two naturally occurring solids would still be referred to as minerals. "Mineral" is the object as a whole.
Whilst the crystals in this quartz and fluorite mineral are large enough to be seen, in other minerals they may not be.
The word "mineral" and "crystal" describe different parts of what is essentially the same thing.
What has made matters more confusing in recent years is the use of the word "crystals" by those who use rocks and minerals for their healing properties. I'll talk more about that later in this article.
A mineral is a naturally occurring solid that forms over millions of years. Quartz exhibits a crystalline structure which means it's made up of crystals.
A crystal refers to the highly ordered repeating arrangement of atoms within a mineral. The crystal is the external shape of the internal arrangement of atoms.
Quartz commonly forms hexagonal prismatic crystals with pointed terminations. These can be seen in the first photo.
Fluorite often exhibits cube-shaped crystals which can be seen in the second photo.
The crystals in these two minerals are clearly visible.
Red jasper is one of many minerals whose crystals can only be seen when viewed through a powerful microscope. The term used to describe these kind of minerals is either microcrystalline or cryptocrystalline.
Crystals are Made Up of Atoms
For a naturally occurring solid to be crystalline (made up of crystals) 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 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 different shapes. This is because of the internal arrangement of atoms.
All crystals exhibit symmetry because they're made up of repeating geometric (regular lines and shapes) patterns.
The following image 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 solidify. This process is called crystallisation. As the arrangement of atoms grow the crystal structure forms. The mass then becomes a crystalline solid.
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. It's 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 virtually everything in the universe are 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. The term mineraloid or amorphous is then used instead.
In an amorphous solid atoms do not come together in a highly ordered arrangement.
Is a Mineral a Rock?
A mineral is an inorganic solid with a crystalline structure (made up of crystals). Inorganic means it does not come from or has been formed by 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 is. That's 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 so they're minerals.
Opal doesn't have a crystalline structure so is classed as a mineraloid or amorphous solid.
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.
The mineral halite forms when saltwater evaporates. The dissolved minerals then crystallise to form a substance used to flavour food.
The cube-shaped crystals of halite are soft and dissolve quickly. They're better known as salt.
Another well known mineraloid is obsidian.
Obsidian forms when a specific type of lava cools on the surface of earth or beneath the ocean. Obsidian is non crystalline because the lava cools too fast for crystals to grow.
A mineral is not a rock because rocks are made up of several different minerals. Rocks made up purely of one mineral do not exist. With that said, some contain about 90% of just one mineral but this kind of rock is relatively rare. Known as monomineralic rocks, the most common are marble, quartzite and limestone.
There are approximately five 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. The presence of impurities often cause a change in colour or introduce different patterns.
Although known for being an exceptionally colourful mineral, in its purest form fluorite is colourless. The colour comes from various impurities.
The deep red colour 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 different minerals that come together through various geological processes. The specific chemical composition determines the type of rock that forms.
Unlike minerals some rocks can be organic meaning they're 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 being bonded 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. 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 expelled from the vent of a volcano are extrusive rocks.
The size of any crystals that form is determined by the length of time it took for the magma or lava to cool and solidify. Where it 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 there's less time for crystal growth. Crystals will therefore either be very small or non existent.
When magma or lava cools and solidifies the minerals in the molten rock crystallise.
Crystallization is the process by which atoms arrange themselves in a repeating, ordered pattern to form a crystal structure.
This is a fundamental process in the formation of minerals.
When minerals crystallise as magma or lava cools, the resulting solid rock is composed of various minerals which are now in their crystalline form. This is why rocks are made up of minerals not crystals.
When magma or lava cools and solidifies too quickly there's no time for crystals to grow. The result is a naturally occurring solid that lacks a crystalline structure. This material is known as "volcanic glass" or an "amorphous solid".
Obsidian is one type of volcanic glass but there are many others. The type that forms depends on a variety of factors.
An amorphous solid (aka mineraloid) describes a naturally occurring solid that's non crystalline. The material will not always be a type of volcanic glass.
An example is opal. This material known for its "play of colour" is classed as an amorphous solid (or a mineraloid) because it's non crystalline. Opal is not volcanic glass.
The rock type I haven't yet mentioned is metamorphic. This material forms 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 several different minerals it occurs when limestone goes through the process of metamorphism. Marble forms when limestone recrystallises.
Lapis lazuli forms in association with metamorphosed limestone but retains its own unique composition and characteristics.
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
In recent years the word "crystals" has become a generic term for rocks and minerals used for their metaphysical healing properties.
It's only used by those who use these materials for healing purposes.
The word "crystals" was popularised by the New Age movement of the 1970s and 80s. It's now widely used around the world in what has become a multi-billion dollar industry.
In the vast majority of cases the word "crystals" is a misnomer and from a geological perspective is often incorrect.
When used to describe rock and mineral-like solids it's not being used literally. It has however created a huge amount of confusion.
The word "crystals" describes a whole host of materials most of which are visually attractive. They may or may not be crystalline in the true sense of the word. In some cases they're even man-made. Examples include goldstone and opalite.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 sugar which is crystallised sucrose and in sand which is crystals of silicon dioxide.
Jewellery is largely made up of crystals. The plaster on walls which is gypsum is crystalline. Tableware such as cups and plates are also produced from crystalline materials.
You may be surprised to learn lead crystal is not crystalline. Despite the name, this material is glass that contains lead. It's not quartz crystal or even a mineral. Due to the lack of any crystalline structure glass is an amorphous solid.
The first picture in our article is the mineral quartz, variety amethyst. The second is fluorite from New Mexico. Both photos along with the mineral halite are courtesy of Stan Celestian.
The red jasper is courtesy of James St.John.
The obsidian was once part of our collection.
The lapis lazuli included with calcite and pyrite is an image we purchased.
The quartz, fluorite, obsidian and halite are all clickable and redirect to the original photos.