The Difference Between Rocks and Crystals
A great deal of confusion exists over the difference between crystals rocks and minerals and the three words are often used incorrectly. These naturally occurring materials are all quite different from each other and hopefully this article will make it easierier to understand exactly what each one is.
If you've ever asked one of the following questions you'll find the answer right here.
- Are crystals rocks?
- What exactly are minerals?
- What are rocks?
If you just want a quick answer then here it is;
Crystals are made up of a highly ordered arrangement of atoms that repeat in a three dimensional pattern so they're not rocks.
Minerals are naturally occurring inorganic solids with a crystalline structure.
Rocks which can be organic or inorganic are made up of several different minerals but can occasionally be made up of just one.
For a more detailed yet simply explained explanation you'll need to keep reading.
If Crystals are Not Rocks What are They?
In recent years the word crystals has been widely used in reference to crystals rocks and minerals particularly when used in conjunction with holistic therapies such as reiki and crystal healing. Stones have been used for thousands of years for metaphysical purposes but use of the word crystals was popularised by the New Age movement of the 1970's and 80's. Today crystals is a multi billion-dollar industry and the practices and therapies associated with them is recognised around the world.
The problem with using 'crystals' as a blanket term for all naturally occurring rock and mineral-like solids is that it's geologically inaccurate and in many cases an incorrect description of what the object actually is. Use of the word generates a great deal of confusion and is used more selectively by mineral dealers, collectors and those with a scientific interest in these materials.
The word crystals is used to describe stones that are rough, polished, tumbled and even those which have been carved into shapes such as hearts, wands, obelisks and pyramids. Goldstone also known as aventurine glass and opalite both of which are man-made materials, are also often referred to as crystals.
Using a quartz crystal as an example, calling this object a crystal is absolutely correct because that's exactly what it is but using the word to describe lapis lazuli or obsidian is incorrect. The reason is because lapis lazuli is a metamorphic rock and obsidian is natural volcanic glass therefore neither material is crystalline.
To really get to grips with the difference between crystals rocks and minerals it's necessary to talk about each one individually.
Cluster of quartz crystals | Photo; Stan Celestian (clickable image)
A Crystal is Made Up of Atoms
For a naturally occurring solid to be correctly 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 a single crystal and the way the atoms come together is known as the crystal structure. There are just seven different crystal structures. To put the size of an atom into perspective, more than one million would fit easily onto a pinhead.
Highly ordered repeating arrangement of atoms
Salt contains crystals shaped like a cube which is one of the most common and simplest shapes found in crystals. This system is known as isometric. A snowflake is made up of a six sided crystal correctly known as hexagonal.
The exterior shape of a crystal reflects the internal arrangement of atoms. The process of crystal growth is called crystallization. Crystals usually form as liquids cool and harden so are basically the shape that minerals take as they cool down and form. For this reason they're sometimes referred to as mineral crystals.
As the arrangement of atoms grows and the crystal structure develops the mass forms a crystalline solid. Crystalline describes a solid made up of crystals.
So crystals are not rocks because they're made up of atoms which in turn are made up of protons, neutrons and electrons but let's not confuse matters any further with that one. Whilst almost everything in the universe is made up of atoms, for the atoms to form a crystalline solid they must come together in a highly ordered repeating pattern. Without making matters unnecessarily complicated, there are many naturally occurring solids whose atoms do not come together in this way which means they're not crystalline or made up of crystals. Glass is one example and obsidian is another.
What is a Mineral is it a Rock?
A mineral is a naturally occurring, inorganic solid with a crystalline structure. Inorganic means it does not come from or has been formed by any living matter. Quartz as an example is a mineral but pearl is not 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 either because it's organic but petrified wood (aka fossilized wood) is because the organic matter in the original organism has been replaced over millions of years with either quartz, calcite, pyrite or occasionally opal. With the exception of opal these materials are all crystalline meaning they're minerals. Opal is one of those naturally occurring solids whose atoms do not come together in a highly ordered repeating pattern.
Minerals form when molten rock (magma or lava) cools either above or below the Earth's surface. They can also form by separating from water that's rich in dissolved minerals. Using halite is an example, as saltwater evaporates the dissolved minerals crystallize to form a substance that's widely used for flavouring food. The crystals which are relatively soft and dissolve quickly are better known as rock salt.
Another example is gypsum which forms from water rich in sulphur and calcium Gypsum is the main constituent in alabaster which has been used for building and sculpting for thousands of years. Plaster and plaster of Paris are both produced from gypsum.
Naturally occurring solids that are void of any significant crystal structure or in other words are not crystalline, are known as mineraloids. These substances include opal, shungite, jet (type of coal), moldavite, pearl and amber (fossilised tree resin) to name just a few. Another mineraloid is obsidian which is natural volcanic glass. It forms as lava cools on the surface of the earth or beneath the ocean. The molten material cools down 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 five thousand different 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 will often contain impurities of other minerals or substances. These impurities can cause a change in colour or introduce various patterns. Fluorite in its purest form is colourless but impurities with help from other geological processes cause it to change colour. The deep red colour of ruby is caused by chromium in the mineral corundum.
Obsidian natural volcanic glass (clickable image)
What Exactly are Rocks?
Rocks are a combination of different minerals or in a few cases just one. They come together through various geological processes and 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 and another is limestone. Sedimentary rocks often contain calcium carbonate shells and skeletons of marine organisms whose remains accumulated as sediment. Over time the sediment becomes buried, compaction gradually squeezes out any water and cementation then leads to layers bonding together to form a material known as sedimentary rock.
The three main types of rock 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 deep below the surface of the earth. This molten rock which is correctly known as magma is a mixture of minerals and dissolved gases. It remains in its fluid state because of immense heat and pressure within Earth's crust. After erupting onto the surface magma becomes known as lava. Rocks which formed as magma cooled and solidified beneath Earth's surface are known as intrusive igneous rocks whilst those formed from lava above the surface are known as extrusive igneous rocks.
Both rock types contain crystals whose size is determined by the length of time that it took for the magma or lava to solidify. Where magma cools over thousands to millions of years crystals have plenty of time to grow so will often be very large. With lava that cools and solidifies rapidly on the surface or beneath the ocean there's less time for growth so crystals will either be small or as in the case of obsidian, almost non existent.
It's important to remember that rocks are produced 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 directly made up of minerals not crystals. To be correctly classified as a mineral a naturally occurring solid must be crystalline and to be crystalline, it must have a highly ordered arrangement of atoms.
The third main type of rock is metamorphic. This material is formed from existing rocks that have undergone a substantial change caused primarily by intense heat and pressure. The process which usually takes place beneath the surface is known as metamorphism which comes from the Latin for 'changed form'.
Marble is a metamorphic rock that was originally limestone but metamorphosed when the calcite recrystallised forming a denser material with similar sized crystals. The different colours found in marble are the result of impurities that were introduced during the transformation.
Earlier in this article I said referring to lapis lazuli as a crystal is incorrect because it's a metamorphic rock. Lapis lazuli occurs in limestone which has been metamorphosed so the material is actually marble. The blue colouration is the caused by impurities of lazurite which is the stone's main constituent but other minerals including pyrite, calcite, sodalite, diopside, amphibole, feldspar, mica and others may also be present.
Lapis lazuli with bands of calcite and pyrite
Not All Rock and Mineral-Like Solids Are Crystals
The purpose of this article was to answer what seemed like quite a straightforward question, are crystals rocks? Whilst it was easy to say no, crystals are not rocks it's also important to understand why.
When 'crystals' is used to describe all rock and mineral-like solids the word is not being used literally. It is instead being used to describe a whole host of naturally occurring (mostly) materials that are visually attractive to the general population and which may or may not be crystalline.
Whilst the rarity of 'visually attractive' or 'pretty crystals' varies widely, it should be remembered that crystals are very common and can be found all around us. In ice which is crystallised water, in salt, in sugar which is crystallised sucrose and sand which is tiny crystals of silicon dioxide more commonly known as quartz.
Granite often used for kitchen worktops is an intrusive igneous rock with large crystals. The plaster on our walls which is made from gypsum is also crystalline. Even tableware such as cups and plates is produced from materials made up of crystals but surprisingly 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.
Using the word quartz with reference to worktops is another misnomer because this material is actually man-made. Although it contains some quartz it's usually just a very small percentage. Other materials present include different types of stone which have been ground up and mixed with resins and a binding agent to give the end product the look and feel of stone.
The point that I'm trying to make is that even when a material is crystalline, it doesn't always make it right to call it a crystal. A fridge is not a crystal even though the metal is crystalline nor is a nail file or sandpaper both of which are likely to be covered in crystals of the mineral corundum. Both ruby and sapphire are gemstone varieties of corundum but the mineral is also widely used as an abrasive.
Are Crystals Rocks? Absolutely Not
I hope that having read this article you now have a clearer understanding regarding the difference between crystals rocks and minerals.