What is a Crystal?
We get the word crystal from the ancient Greek word krystallos which roughly translates into “coldness drawn together.” In one aspect, that is completely correct, in scientific terms, crystals represent the “frozen” or cold version of liquids and gasses.
What defines a crystal? It is a body of uniform material with a regular internal structure, an organized internal structure with a regular, repeating arrangement of the small particles of atoms, ions or molecules. This arrangement is called a crystal lattice. The study of crystals is called crystallography.
The shape of a crystal is defined by its crystal lattice. There are seven main categories of crystal lattice shapes called crystal systems based on the shape and orientation of the lattice axes. Five of the systems are defined by three primary axes, these include: cubic, tetragonal, orthorhombic, monoclinic, and triclinic. The other two systems, hexagonal and trigonal include a fourth axis.
Cubic System (three axes) - All three axis are of equal length and are at right angles to each other.
Tetragonal System (three axes) - Two axis are of equal length and lie in a horizontal plane at right angles. The third axis, the principal axis, is either longer or shorter than the other two and is oriented vertically, perpendicular to the other axes.
Orthorhombic System (three axes) - All three axes are of different lengths and are orientated at right angles to each other.
Monoclinic System (three axes) - Two axis are of equal length and are oriented at right angles to each other. The third axis is either shorter or longer and is situated at an irregular angle.
Triclinic System (three axes) - All three axes are of different lengths and are oriented at irregular angles to each other.
Hexagonal System (four axes) - Three of the four axes lie in one plane. Those three axes are of equal length and intersect at angles of 120 degrees. The fourth axis is not the same length and is perpendicular.
Trigonal System (four axes) - Three of the four axes lie in the same plane and are of equal length and intersect at angles of 120 degrees. The fourth axis is not the same length and is perpendicular. The only difference compared to a hexagonal system is that the corners of the crystal are beveled off.
Typical ice formed at standard pressure has a Hexagonal System
Tourmaline
1700s
Tourmaline first gained scientific attention in Europe during the early 1700s. Dutch traders, who brought the gemstone from Sri Lanka (then Ceylon), nicknamed it "Aschentrekker" or "ash puller."
The "Magic" Trick: They noticed that when the stone was placed near hot coals, it would first attract and then repel hot ashes.
The Physics: This was the first documented observation of pyroelectricity. Heating the crystal causes its ends to become oppositely charged, allowing it to attract particles like dust or ash.
1800s
Swedish botanist Carl Linnaeus officially dubbed it "the electric stone."
In 1880, Pierre Curie and his older brother, Jacques, were investigating whether mechanical stress—not just heat—could produce electricity in crystals.
The Experiment: The Curie brothers used a slab of tourmaline clamped between two electrodes and connected to an electrometer. They discovered that applying mechanical pressure to the crystal generated a measurable electric charge.
Naming the Effect: This phenomenon was named piezoelectricity, derived from the Greek word piezein, meaning "to press."
Significance: While they also tested other crystals like quartz and topaz, tourmaline was one of the primary materials used to prove that a crystal's lack of internal symmetry (its "dissymmetry") is what allows it to convert mechanical energy into electrical energy.
The Converse Effect: A year later, they confirmed that the process works in reverse: applying an electric field to a tourmaline crystal causes it to physically deform.
Piezoelectricity (Pressure): When you squeeze a tourmaline crystal, you physically push these charged ions (like silicon and oxygen) closer together or further apart. Because the structure is asymmetrical, these ions can't just shift and stay balanced; their movement shifts the center of the positive and negative charges, creating a measurable voltage on the surface.
Pyroelectricity (Heat): Heating causes the entire crystal lattice to expand. As the atoms move apart, the distance between the "tips" of those silicon tetrahedra changes. This alters the strength of the internal dipole, causing electrons to rush to one end of the crystal to try and balance the change.
Tourmaline's ability to generate electricity is rooted in its unique, asymmetrical molecular structure. Unlike many minerals that are balanced, tourmaline is inherently "lopsided" at the atomic level.
1. The Polar Symmetry (Space Group R3m)
In crystallography, tourmaline belongs to the Trigonal crystal system and specifically the 3m point group.
Non-centrosymmetric: Most importantly, it lacks a "center of symmetry." If you were to start at the center of the crystal and move in one direction, you wouldn't find a matching atom at the same distance in the exact opposite direction.
The Polar Axis: Because of this lack of symmetry, the crystal has a unique polar axis (the c-axis). The top of the crystal is fundamentally different from the bottom.
The Dipole Moment: Crucially, all these tetrahedra point their "tips" in the same direction along the c-axis. This creates a permanent internal electric dipole—like a tiny, built-in battery—even when the stone is just sitting still.
Crystallographers: Jenny Pickworth Glusker
Dorothy Crowfoot Hodgkin
Isabella Karle
Jane RIchardson