Is a Crystal a Rock?
| Contents 1. Crystals are Not Rocks 2. What are Crystals? 3. Crystals Made Up of Atoms 4. Is a Mineral a Rock? 5. More About Rocks 6. The Word 'Crystals' 7. Article Pictures |
Crystals are Not Rocks
The difference between crystals, rocks, and minerals always causes so much confusion.
These three naturally occurring solids are not all the same, so I hope this article will help you understand what sets each one apart.
In this article, I'll answer the following questions:
- Are crystals rocks?
- What are minerals?
- What are rocks?
If you're looking for a quick answer, here it is:
Crystals are made up of atoms, so they're not rocks.
Minerals are composed of crystals, so they're not rocks either.
Rocks are formed from a combination of several different minerals.
For a more detailed explanation, read on.
So, What are Crystals?
The following two photos are 'crystals' in the geological sense of the word. The mineral in the first photo is quartz, and the second is fluorite.
Although crystals are the most obvious feature, these two naturally occurring solids would still be called minerals. A 'mineral' is the object as a whole.
While the crystals are large enough to be seen, in other minerals they may not be.
The words 'mineral' and 'crystal' describe different parts of what is essentially the same thing.
In recent years, the use of the word 'crystals' by those who use rocks and minerals for their healing properties has added to the confusion, which I'll discuss later in this article.
A mineral is a naturally occurring solid that forms over millions of years. For example, quartz is a mineral that exhibits a crystalline structure, meaning it's made up of crystals.
A crystal is a 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, as seen in the first photo.
Fluorite often exhibits cube-shaped crystals, as seen in the second photo.
Red Jasper is one of many minerals whose crystals can only be seen through a powerful microscope. These types of minerals are correctly known as microcrystalline or cryptocrystalline.
Crystals 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, each defined by the internal arrangement of atoms. This internal atomic pattern determines the external shape of a crystal.
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 that a crystal takes is known as its habit. Cube-shaped crystals, also known as isometric, are among the most common and simplest shapes. Salt forms isometric crystals, while a snowflake is composed of hexagonal, six-sided crystals.
Crystals often (but not always) form when liquids cool and solidify. This process is called crystallisation. During crystallisation, atoms and molecules arrange themselves into a highly ordered, repeating pattern as the substance changes from a liquid to a solid. This organised arrangement leads to the formation of a crystalline solid.
Crystals are made up of repeating units of atoms arranged in a specific pattern, known as unit cells. A single crystal can consist of just a few or billions of unit cells.
The arrangement of the unit cells forms a regular 3D structure called the crystal lattice. The shape of the unit cell and the symmetry of the lattice determine the external shape of the crystal's faces.
All states of matter, which include gases, liquids, and solids, are made up of atoms. For a solid to be crystalline, the atoms must be arranged in a highly ordered, repeating pattern. If this pattern is not present, the substance is non-crystalline.
Non-crystalline substances are often referred to as mineraloids. The word 'amorphous' may also be used, however, it has a slightly different meaning.
Is a Mineral a Rock?
A mineral is an inorganic solid with a crystalline structure. Inorganic means it does not come from or has been formed by living matter.
Quartz and fluorite are minerals, but pearl is not. Pearl is 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 does not have a crystalline structure, so it's a mineraloid.
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 salt water evaporates. The dissolved minerals then crystallise to form a substance that's used to flavour food.
These cube-shaped crystals, which dissolve in water, are better known as salt.
A mineral is not a rock because rocks are generally composed of several minerals. However, some are made up almost entirely of just one mineral. These rocks are known as monomineralic.
Examples of monomineralic rocks include marble (composed mainly of calcite), quartzite (composed mainly of quartz), and limestone (also composed mainly of calcite). Monomineralic rocks are relatively rare.
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, amphibole, micas, olivine, garnet, calcite and pyroxenes.
Minerals often contain traces of other minerals or substances. These impurities can bring about a change in colour. The purple colour of amethyst is primarily due to trace amounts of iron.
An impurity is not the same as an inclusion.
More About 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 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.
Some types of limestone are also organic.
Sedimentary rocks often contain shells and skeletons of marine organisms whose remains accumulate as sediment. Over time, the sediment becomes buried, compaction squeezes out any water, and cementation gradually binds layers 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 above or below Earth's surface. Molten rock is a mixture of minerals and dissolved gases. Because of immense heat and pressure, it remains fluid.
Rocks that formed as magma cooled and solidified beneath the Earth's surface are known as intrusive igneous rocks. Those formed from lava expelled from the vent of a volcano are extrusive rocks.
The size of crystals that form is determined by the length of time that it takes 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, there is less time for crystal growth. Therefore, crystals will either be very small or non-existent.
This rapid cooling results in a naturally occurring solid that lacks a crystalline structure. This material is volcanic glass, which is a mineraloid.
Obsidian is one type of volcanic glass, but there are many others. The specific type of glass that forms depends on various factors, including the composition of the lava or magma and the cooling conditions.
Not all mineraloids are volcanic glass. Opal, known for its play of colour, is also a mineraloid, and so is shungite.
When magma or lava cools and solidifies, the minerals in the molten rock crystallise. A mineral only begins to form when atoms organise themselves into a highly ordered, repeating pattern, creating the crystal structure.
Once the minerals have crystallised, the rock that forms is composed of these minerals in their crystalline form. This is why rocks are made up of various minerals, each with its own crystal structure, rather than being composed solely of crystals.
I haven't spoken about metamorphic rocks in this article but have written about them in a separate blog post. You can read it here.
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 not used by geologists or those whose interest in these materials is scientific.
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 many cases, the word is a misnomer and from a geological perspective, is often incorrect. However, when used to describe rock and mineral-like solids, it's not being used literally, however, it has created a considerable amount of confusion.
The word 'crystals' is used to describe 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 and, some are even man-made. Examples include goldstone and opalite.
While the rarity of certain rocks and minerals varies widely, crystals in the true sense of the word are extremely common. Crystalline substances that everyone's familiar with include ice, which is crystallised water, sugar which is crystallised sucrose, and sand which is crystals of silicon dioxide.
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You may be surprised to learn that lead crystal is not crystalline. Despite the name, this material is a type of glass that contains lead. Because glass lacks a crystalline structure, it's as an amorphous solid.
The term 'amorphous' is used instead of 'mineraloid' because lead crystal is man-made. 'Amorphous' shouldn't be used to describe non-crystalline solids that have occurred naturally.
Article Pictures
Quartz crystals var. amethyst, fluorite, halite - Courtesy of Stan Celestian.
Red Jasper - Courtesy of James St.John.
Photos are clickable and redirect to the original image.
"How to Identify Rocks" courtesy of Bill Langer, U.S Geological Survey.
Pop-up photos: Quartz, amphibole (on calcite) - Courtesy of Ron Wolf. Fluorite - Courtesy of Steve Blyskal.
Calcite, pyrite, opal, marble, limestone, olivine - Courtesy of Stan Celestian.
Quartz - Courtesy of Lawrence Violett.
The remaining pieces in pop-up photos come from our collection.
