What is Crystal in Physics Actually (Shops Hate this!)

What is crystal in physics is different than what people commonly refer to crystals. People will call basically any rock a crystal, especially one that looks nice. That’s fine, but in physics the reason crystals are distinct are because of what they can do.

To be a crystal by the physics definition, the molecules have to have long range order. On a small scale, you can take one unit of the chemical structure, and tessellate, or tile it, in 3D in all directions until you reach the edge of the material.

What is Crystal in Physics Defined

In physics, crystals have defined structure, which means the chemical formula is also clearly defined throughout the entire material. The way the bonds between the atoms and their arrangements are predictable and constant. Because there is a consistent chain of atoms with identical links, energy can travel in a computable way for long enough to really measure and make use of. That’s what’s special about long-range order in crystalline materials.

Crystals in physics can be very neatly categorized. Even though there are a huge amount of examples, there are only a finite number of ways molecules can bond that’s regular. There are 7 crystal systems, which set up the coordinate grid you need to work within to study the crystal. The systems are further broken down into classes, which tell the specific symmetry within the crystal. Even crystals of the same class can have variation in their effect of course, with the building blocks swapped out with different elements.

Name of Crystal System	Restrictions on unit cell axis lengths and angles
triclinic	x ≠ y ≠ z and α ≠ β ≠ γ
monoclinic	x ≠ y ≠ z and α = γ = 90 ≠ β
orthorhombic	x ≠ y ≠ z and α = β = γ = 90
tetragonal	x = y ≠ z and α = β = γ = 90
trigonal	x = y ≠ z and α = β 90 and γ = 120
hexagonal	x = y ≠ z and α = β 90 and γ = 120
cubic	x = y = z and α = β = γ = 90

Crystals in Shops and Common Usage

The crystals in the shops are usually amorphous rocks and minerals. They may have many different stripes of chemical formulas, making them texturally interesting. Or they may really be what is a crystal in physics. Here are some common examples.

“Real” Physics crystal	Crystal Shop Mineral
Pyrite	Labradorite
Hematite	Obsidian
Tourmaline (many different species)	Tiger’s Eye
Zircon	Shungite
Calcite	Bloodstone
Clear Quartz	Types of colored quartz such as citrine, amethyst, etc.
Fluorite	Carnelion
Turquoise	Lapis Lazuli

Some of the amorphous minerals have striations of crystalline formations. From an energetic standpoint, even in physics, this makes the stone potentially more interesting too. This is because an energetically active crystalline material is even more energetically active because it has more exposed surface area. The surface is where the material takes in and outputs energy in the environment. So a powder is more active than a sheet, which is more active than a mass of it, for any given type. The breaks in symmetry from striations make the stone just more complex and harder to use in applications.

So just because the crystals in shops aren’t what is a crystal in physics makes them no less interesting. For example, many of the amorphous stones have inclusions of crystalline material that have cool magnetic properties, or applications in industrial computing. If anything, that’s neat to own something both beautiful and represents the potential it naturally holds.

What qualifies as a real crystal?

All the above “physics crystals” have a repeating regular pattern to their molecular arrangement. So here are the chemical formulas and visuals to go with that.

Physics Crystal	Crystal system	Visual representation
Hematite (Fe2O3)	Trigonal
Calcite (CaCO3)	Trigonal
Fluorite (CaF2)	Cubic
Diamond (pure Carbon)	Cubic
Quartz (SiO2)	Trigonal

Another example is turquoise, CuAl₆(PO₄)₄(OH)₈·4H₂O, which has a triclinic crystal system.

Often in classes of materials, like “silicates,” there is more specific breakdowns, until you get to the crystal which is more or less a random inclusion in the broader category. For example, within silicates there is pyroxenes, mica, etc. Within mica there is a number of crystal “species” like muscovite, etc. Eventually you get a specific species that has a clearly defined formula and thus structure. The amorphous materials are defined by their impurities and inclusions.

What is Crystal in Physics is not Always Crystal in Ordinary Speech

So anyone is free to use the word crystal to describe any rock. But remember if a scientist uses the word, they are not talking about any rock most likely. And when you own a colloquial crystal that happens to be a “real” physics crystal as well, you can look up the chemical formula and find out what it looks like on a small scale.