How do crystals form?

Crystals have always intrigued humanity, from the famous Hope diamond to the sparkling particles in Folgers coffee. These fascinating forms have inspired seers and adorned emperors' crowns for centuries. But they're not just beautiful objects; crystals boast practical qualities. They fortify metals, power our watches, and illuminate our modern world through digital screens and fluorescent bulbs.

And don’t forget, they also enhance the flavor of our food and chill our drinks.

Indeed, salt, sugar, and ice are crystals, just like the jewels, metals, fluorescent paints, and liquid crystals we’ve mentioned. That’s part of their appeal: crystals can be made from almost anything. In fact, most minerals naturally take on a crystalline form [source: Smithsonian].

A clue to why crystals are so pervasive can be found in our daily language. When we say someone's thoughts suddenly 'crystallize' around a solution, we all immediately understand what that means: a chaotic array of possibilities has formed into something neat and orderly. Whether we realize it or not, we recognize that the defining feature of a crystal is order – specifically, a consistent, periodic arrangement of atoms [source: UCSB].

Crystals can form anywhere, from a kitchen pie tin to a state-of-the-art lab, or even deep within the Earth. The process is surprisingly straightforward: start with a cloud of gas, a pool of solution, or molten rock, then overload it with the right mineral or compound, and cook it under pressure somewhere between room temperature and the heat of molten lava. But turning this recipe into reality can require the precision of a skilled chef and the exacting control of a master baker – or, in the case of natural crystals, just a lot of time and a bit of luck [sources: Hunting; Shea; Smithsonian].

In general, longer growth periods yield larger crystals with fewer impurities [sources: CU Boulder; UCSB]. But sometimes, the impurities are essential: it’s elements like chromium, iron, and titanium – along with variations in atomic structure – that give gems their unique colors [sources: Encyclopaedia Britannica; Kay; Smithsonian].

Naturally, crystals, like anything, need space to grow. If they're confined to tight spaces, they'll remain small; pack several crystalline minerals into a small area, like sardines in a subway car, and you get crystal conglomerates. Granite, the go-to rock for tombstones and countertops, is a conglomerate of quartz, feldspar, and mica crystals that form as magma cools in the narrow crevices of volcanic fissures [source: Smithsonian].

And that's it: the process of growing a crystal.

Wait, what exactly is a crystal again?

What Are Crystals?

In physics, a 'crystal' refers to a solid material that possesses internal symmetry and a consistent, repeating surface pattern. This regular structure, known as the crystal structure, repeats so predictably that it allows one to determine the atomic arrangement throughout the crystal [sources: Encyclopaedia Britannica; Isaacs et al.].

If this arrangement extends beyond a few neighboring atoms, it is called long-range order, similar to a marching band performing at halftime. On the other hand, liquid crystals, such as those in LCD screens, typically display short-range order (imagine the band splitting into smaller sections during their performance). Solid crystals can follow either pattern. Here's how: When crystalline materials melt, they become amorphous, meaning they only show short-range order. As they cool, they may either return to long-range order or remain amorphous, like silicon-based glass [sources: Arfken et al.; Encyclopaedia Britannica; Isaacs et al.].

In this analogy, ions (atoms with either a positive or negative charge) play the role of our band members, connected by ionic or covalent bonds. These bonds form into various compact and stable structures known as coordination polyhedra [sources: Banfield; Dutch].

To better visualize these coordination polyhedra, forget about the marching band and instead imagine a geometric mosaic, like those found in the Alhambra. Now, envision that mosaic in three dimensions, with the tesserae (tiles) made of cubes, pyramids, and diamond-shaped solids, each representing the atomic arrangement in a particular crystal type.

In a silica crystal, a small central silicon ion may be surrounded by four larger oxygen ions, forming a tetrahedron (a triangular pyramid). In manganese(II) oxide, a central manganese ion is surrounded by six oxygen ions—one above, one below, and four in a square around the center, forming a three-dimensional octahedron (diamond shape) [sources: Banfield; Dutch; Purdue].

These three-dimensional mosaic tiles can organize into different patterns, or lattices, where atoms share bonds at their corners, along edges, or across faces. The same elements can adopt various arrangements, both in their 'tile shapes' (coordination polyhedra) and their mosaic structures (lattices). These variations, known as polymorphs, are key to determining a crystal's properties. For example, carbon arranged tetrahedrally forms the famous hard, clear diamond, while in a layered honeycomb structure, it forms soft, gray graphite [sources: Dutch; Purdue; UCSB].

Crystallization doesn't always lead to single crystals. Sometimes, the self-organizing process starts at multiple sites, growing together to form a patchwork of lattices aligned in different directions. These polycrystals, which often form during rapid cooling, tend to be stronger than single crystals [sources: Encyclopaedia Britannica; Encyclopaedia Britannica; University of Virginia]. When heated, larger crystals can absorb smaller ones. Temperature, pressure, stress, and strain all play a role in shaping a crystal’s characteristics, whether in its transformation or its formation.

Crystal Blue Persuasion

If the idea of growing crystals yourself intrigues you, you're in luck, or maybe not, depending on what you wish to grow. Salt or sugar? Sure. Artificial diamonds? Well, even villain Blofeld found it easier to simply smuggle them than grow them.

There are three main methods to grow crystals: from vapor, from a solution, or from melt. Let's dive into each method, starting with vapor deposition.

It shouldn't be surprising that crystals can form from vapor. After all, atmospheric ice crystals -- known to us as clouds and snowflakes -- do this constantly. They build up when the atmosphere becomes supersaturated with moisture: It holds more water than it can manage at a given temperature and pressure, forcing the excess to leave the gaseous state and crystallize into ice [sources: Encyclopaedia Britannica; Libbrecht].

Other crystal types, such as silicon, can also form from gases supersaturated with essential elements, though they may require a little chemical assistance to begin the process [sources: Encyclopaedia Britannica; McKenna].

Typically, the growth process starts with a tiny seed crystal, onto which molecules attach one by one as they emerge from suspension -- much like how silver iodide crystals assist in "cloud seeding" by providing nucleation points for ice crystals. It requires patience, but this method results in remarkably pure crystals [sources: Encyclopaedia Britannica; McKenna].

Solution-based growth shares many similarities with vapor-based growth, except that liquid replaces gas as the supersaturated medium. Crystals like salt and sugar, commonly created as science experiments, are examples of solution-grown crystals. This method outpaces gas deposition in both growth speed and size. Here's why: In a gas, the vaporized substance dances among other molecules in a chaotic waltz, making it take longer for individual particles to leave and form crystals. A solution, on the other hand, behaves more like a slow dance, where crystalizing molecules linger near the surface, promoting quicker growth. This ease of use explains why the solution method dominated synthetic crystal growth for many years [sources: Encyclopaedia Britannica; Zaitseva et al.].

The third method, growth from melt, involves first cooling a gas into a liquid and then chilling the liquid until it solidifies into a crystal. This method is great for creating polycrystals but can also produce single crystals using techniques such as crystal pulling, the Bridgman method, and epitaxy. We'll examine each of these methods in the next section [source: Encyclopaedia Britannica].

I'll Melt With You

In the past, growing crystals from melt was as much an art form as a science. Today, it's a precise process that utilizes advanced techniques to control growth conditions, sometimes even down to the molecular level.

In crystal pulling, a machine slowly lowers a seed crystal until it just touches a blob of molten material, and then gradually moves the seed upward in sync with the crystal's growth rate. Altering the speed of movement affects the crystal's size. This method is used to grow the large silicon crystals used in computer chips, fittingly controlled by computers themselves. It's like the silicon circle of life.

The Bridgman method involves using a crucible, a specially designed container with a tapered bottom, to hold molten material. The crucible is slowly lowered into a cooler region, initiating crystal growth at the tip, which then extends upward as the crucible descends further. This back-and-forth process ensures that the crystal formation area stays within an optimal temperature range for growth, eventually resulting in a single, complete crystal [sources: Encyclopaedia Britannica; Chen et al.; Yu and Cardona].

Epitaxy (derived from the Greek words epi meaning "upon" and taxis meaning "arrangement") highlights a method where crystals are grown on top of existing ones. However, not all crystals are suitable for this purpose. The base, or substrate, must be perfectly flat, even at an atomic level. Additionally, to ensure proper crystal growth, the substrate's structure should closely resemble the desired lattice of the crystal being formed [sources: Encyclopaedia Britannica; Fang et al.; Oxford Dictionaries; Yu and Cardona]. Imagine a billiard table with balls arranged in a rack, then adding more balls on top. The new balls will settle into the spaces between those beneath them.

Epitaxy is an umbrella term for various crystal growth techniques [sources: Encyclopaedia Britannica; Yu and Cardona]:

Alright, enough about consumer electronics. We all know that it doesn't really count if you don't have that bling.

Famous Crystals I Have Known

Crystal Gayle, Crystal Bernard, Crystal the Monkey -- no, we aren't talking about any of them. When we mention famous crystals, we're of course referring to precious gems. Sparkling jewels. Glimmering rocks. The kind that catch the light and steal the show.

Jewels.

Gemstones are crystals with a little extra flair. Call it charm, or sparkle. While we tend to think of gemstones as unique stones, many actually come from the same mineral sources. The real difference lies in the structural quirks and mineral impurities that give each one its signature hue.

Rubies and sapphires are both varieties of corundum (crystalline aluminum oxide, or alumina), but they differ in color due to the specific impurities they contain. Rubies get their rich red hues from small amounts of chromium replacing aluminum in the crystal structure, while sapphires owe their brilliant blue shades to iron and titanium impurities [sources: Encyclopaedia Britannica; Kay].

Amethyst and citrine are two types of quartz (crystalline silicon dioxide, also known as silica), a mineral that is naturally colorless. The ancient Greeks believed quartz was ice that had frozen so solidly it couldn’t melt, which led them to call it krystallos ("ice"). This is where we get the word crystal. Citrine's yellow tint comes from amethyst that's been heated, while the exact cause of amethyst's purple color is still debated. Some say it's due to iron oxide, others point to manganese or hydrocarbons [sources: Banfield; Encyclopaedia Britannica; Encyclopaedia Britannica].

The silicate mineral family, rich in silica, includes stones like tourmaline, prized both as a gemstone and for its piezoelectric properties, and beryl, a family of gems like aquamarine (pale blue-green), emerald (deep green), heliodor (golden yellow), and morganite (pink). The largest crystal ever discovered was a beryl from Malakialina, Madagascar. It measured an impressive 59 feet (18 meters) in length and 11 feet ( meters) across, weighing a massive 400 tons (380,000 kilograms) [sources: Banfield; Encyclopaedia Britannica; Encyclopaedia Britannica].

Silicates are just one type of elemental crystal family. Oxides (such as corundum) contain oxygen as a negatively charged ion; phosphates are rich in phosphorus; borates feature boron (B); sulfides and sulfates are teeming with sulfur; and halides include chlorine and other elements from group VIIA of the periodic table [source: Banfield].

The carbonate family is composed of crystals rich in carbon and oxygen, best known in the jewelry world for aragonite, a variety of calcium carbonate used by oysters to form pearls. Aragonite can be produced through either geological or biological processes [sources: Banfield; Encyclopaedia Britannica].

In the Mexican state of Chihuahua, there is a remarkable limestone cavern known as the Cueva de los Cristales, or the Cave of Crystals. This cave is filled with enormous, transparent crystals of selenium (a variety of transparent gypsum), some reaching an astounding 30 feet (9 meters) in length, which easily outsize human explorers [source: Shea].

So, what's the largest crystal in the world? It might just be Earth's very own -- literally. Some scientists suggest that the Earth's inner core, which is roughly the size of the moon, could actually be one enormous iron crystal [source: Broad].