From the Hope diamond to the shiny bits in Folgers coffee, crystals have always held the power to fascinate, inspiring soothsayers and adorning the crowns of emperors throughout history. But crystals aren't just a bunch of pretty facets -- they glimmer with useful properties. They lend strength to worked metals, run our timepieces and drive the digital displays and fluorescent bulbs of modern life.
Oh, and they season our food and cool our beverages as well.
Yes, salt, sugar and ice are crystals, too, just like the gems, metals, fluorescent paints and liquid crystals we mentioned. That's part of their charm; crystals can be made from just about anything. In fact, most minerals naturally occur in a crystalline form [source: Smithsonian].
A clue to this ubiquity can be found in our everyday speech. When we say someone's thoughts suddenly "crystallize" around a solution, we're all crystal clear on what that means: that a jumble of swirling possibilities resolved itself into something still and orderly. Consciously or not, we understand that the essential quality of a crystal is order -- specifically, a regular, periodic arrangement of atoms [source: UCSB].
Crystals can grow in a countertop pie tin, a high-tech lab or a fissure deep in the Earth. The recipe is deceptively simple: Take a cloud of gas, a pool of solution or a glop of melted rock, overstuff it with the right mineral or compound, then bake in a pressure cooker at somewhere between room temperature and the heat of molten lava. But executing that recipe can require the artistry of a chef and the meticulous control of a master baker -- or, in the case of natural crystals, dumb luck and an awful lot of time [sources: Hunting; Shea; Smithsonian].
All else being equal, longer growth times produce bigger crystals with fewer contaminants [sources: CU Boulder; UCSB]. Not that you always want to lose the impurities: After all, it's intruders like chromium, iron and titanium -- along with aspects of atomic arrangement -- that give gems their characteristic colors [sources: Encyclopaedia Britannica; Kay; Smithsonian].
Of course, crystals, like anything else, need room to grow. Trap them in cramped quarters and they stay small; jam several crystalline minerals into a small space like Japanese subway commuters, and you end up with crystal conglomerates. Granite, the favored rock of tombstones and countertops everywhere, is a conglomeration of quartz, feldspar and mica crystals, which grow as magma cools in cramped volcanic fissures [source: Smithsonian].
So there you have it: how to grow a crystal.
Now ... what was a crystal again, exactly?
What Are Crystals?
In physics, the term "crystal" describes a solid substance with internal symmetry and a related, regular surface pattern. This configuration, called the crystal structure, recurs so regularly that you can use it to predict the organization of atoms throughout the crystal [sources: Encyclopaedia Britannica; Isaacs et al.].
If this arrangement carries on beyond a few neighboring atoms it is called long-range order, akin to a half-time band marching in formation. Liquid crystals, like those found in LCD monitors, usually fall into short-range order (picture the marching band scatter-drilling into smaller subunits). Solid crystals can assume either pattern. Here's how: As crystalline substances melt, they become amorphous, meaning they display only short-range order. As they cool, they can either fall back into a long-rage formation or remain amorphous, like silicon-based glass [sources: Arfken et al.; Encyclopaedia Britannica; Isaacs et al.].
Cast in the role of our band members are ions (positively or negatively charged atoms) linked up by ionic or covalent bonds. These bonds pack up into various compact, stable shapes called coordination polyhedra [sources: Banfield; Dutch].
To better picture these coordination polyhedra, forget the marching band and instead picture a geometric mosaic like those found in the Alhambra. Now visualize that mosaic in three dimensions so that its tesserae (tiles) consist of cubes, pyramids and diamond-shaped solids, each of which describes the arrangement of the atoms in a given type of crystal.
In a silica crystal, a small central ion of silicon might be surrounded by four larger ions of oxygen, forming a triangular pyramid, or tetrahedron. In manganese(II) oxide, a small central manganese ion lies within six larger oxygen ions -- one above, one below and four in a square around the middle, forming a three-dimensional diamond, or octahedron [sources: Banfield; Dutch; Purdue].
These 3-D mosaic tiles can pack into several different patterns, or lattices, sharing atomic bonds at their corners, along their edges or along their faces. The same elements can assume different arrangements, both in terms of their "tile shapes" (coordination polyhedra) and their mosaic patterns (lattices). These variations are called polymorphs, and they play a key role in determining a crystal's properties. Take carbon: Arranged tetrahedrally, it forms famously hard, clear diamonds; arranged in a layered honeycomb, it forms soft, gray graphite [sources: Dutch; Purdue; UCSB].
Crystallization doesn't always produce single crystals. Sometimes, the self-ordering process begins at a number of sites that grow together, forming a patchwork of lattices aligned along different directions. These polycrystals, which often develop 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. So temperature and pressure, stress and strain can influence crystals' characteristics, whether in their transformation -- or their creation.
Crystal Blue Persuasion
If all this talk of crystals has you itching to grow some yourself, you're in luck -- or not, depending on what you want to grow. Salt or sugar? Sure. Artificial diamonds? You'll soon see why even Bond villain Blofeld decided it was simpler just to smuggle them.
You can grow crystals in one of three major ways: from a vapor, from a solution or from melt. Let's look at each method one by one, beginning with vapor deposition.
The fact that crystals can grow from a vapor should come as no surprise. After all, atmospheric ice crystals -- we call them clouds and snowflakes -- do it all the time. They accumulate because the atmosphere becomes supersaturated with moisture: It contains more water than it can hold at a given temperature and pressure, so excess water leaves the gaseous state and aggregates into crystalline ice [sources: Encyclopaedia Britannica; Libbrecht].
Other crystal types -- silicon, for example -- can grow from gases supersaturated with key elements, but might need a little chemically reactive boost to do so [sources: Encyclopaedia Britannica; McKenna].
In most cases, the process begins with a tiny seed crystal to which other molecules attach, layer by layer, as they come out of suspension -- much in the way silver iodide crystals aid in "cloud seeding" by providing nucleation sites for ice crystals. The process requires great patience, but it produces surprisingly pure crystals [sources: Encyclopaedia Britannica; McKenna].
Growth from solution shares much in common with vapor growth, but liquid replaces gas as the supersaturated medium. Salt and sugar crystals created as science projects are good examples of solution-grown crystals. The solute approach outperforms gas deposition in terms of both growth speed and crystal size. Here's why: In a gaseous state, the vaporized substance whirls in a dizzy Viennese waltz among other gas molecules, and it can take a while for individual dancers to leave the floor and join the crystalline clique. A solution acts more like a high school slow dance, complete with crystalizing wallflowers that hang out near the surface, promoting faster growth. Its user-friendliness explains why the solution approach dominated synthetic crystal growth for many years [sources: Encyclopaedia Britannica; Zaitseva et al.].
The third method, growth from melt, requires first cooling a gas to a liquid state and then chilling the liquid until it attains crystalline solidity. The melt method excels at making polycrystals but can also grow single crystals using techniques such as crystal pulling, the Bridgman method and epitaxy. Let's take a closer look at each in the next section [source: Encyclopaedia Britannica].
I'll Melt With You
Historically, growing crystals from melt was as much art as science. Today, it entails any one of a number of high-tech techniques that meticulously control growth conditions, sometimes at the molecular scale.
In crystal pulling, a machine lowers a seed crystal until it just kisses a glob of melt, then gradually moves the burgeoning seed upward, timing its motion to coincide with the crystal's growth rate. Changing the movement rate alters the crystal's diameter. Manufacturers grow the large-diameter silicon crystals found in computer chips this way -- which seems appropriate, since computers also control the pulling process. Think of it as the silicon circle of life.
Under the Bridgman method, manufacturers take a crucible (a specialized container used to heat substances) with a conical lower end, fill it with molten material, then lower it into a cooler region. Crystal growth kicks off at the cooled crucible tip, then works its way up as the crucible continues downward. Thanks to this coming-and-going approach, the crystal formation area remains within a growth-friendly temperature zone until, finally, the crucible's contents form a single crystal [sources: Encyclopaedia Britannica; Chen et al.; Yu and Cardona].
Epitaxy (from Greek epi "upon" + taxis "arrangement") reminds us that sometimes the best way to grow a crystal is on top of another crystal. Not just any crystal will do, however. First, the base, or substrate, must be quite flat, even at the atomic scale. Second, because the substrate's structure strongly influences the atomic arrangement of the growth crystal, it should match the desired growth lattice as closely as possible [sources: Encyclopaedia Britannica; Fang et al.; Oxford Dictionaries; Yu and Cardona]. Picture a full rack of billiard balls and then imagine stacking more balls on top it. You can move the new balls around, but they always end up seated in the hollows between the balls beneath.
- Molecular beam epitaxy (MBE), for example, grows crystals layer by layer using beams of molecules.
- Synthetic diamond manufacturers rely on chemical vapor deposition (CVD), a faster approach that trades the beam in favor of a flowing gas.
- Crystals slated for electronics rely on liquid-phase epitaxy (LPE), in which a crystal grows on a substrate situated within a saturated solution.
OK, that's enough talk about consumer electronics. We all know that it don't mean a thing if you ain't got that bling.
Famous Crystals I Have Known
Crystal Gayle, Crystal Bernard, Crystal the Monkey -- no, we don't mean any of those. When we speak of famous crystals we are, of course, referring to bling. Ice. Rocks. Fist sparklers.
Gemstones are crystals with a certain extra something. Call it pizzazz. Although we tend to think of them as individual rocks, many gemstones arise from the same minerals. The only differences between them are the structural idiosyncrasies and mineral impurities that imbue them with their trademark colors.
Rubies and sapphires are both types of corundum (crystalline aluminum oxide, or alumina), but while ruby's luscious reds derive from tiny amounts of chromium that partially replace aluminum in the crystal structure, sapphire's brilliant blues come from iron and titanium impurities [sources: Encyclopaedia Britannica; Kay].
Amethyst and citrine are different versions of quartz (crystalline silicon dioxide aka silica), which is naturally colorless. Ancient Greeks thought quartz was ice that had frozen so hard it wouldn't melt, so they called it krystallos ("ice"), thereby giving us the word crystal. Yellowish citrine arises from overheated amethyst, but experts differ over what precisely gives amethyst its characteristic purple pop. Some say it's iron oxide, while others favor manganese or hydrocarbons [sources: Banfield; Encyclopaedia Britannica; Encyclopaedia Britannica].
The silica-rich mineral family, or silicates, includes tourmaline, valued both as a gemstone and for its piezoelectric properties, and beryl, a family of gems comprising aquamarine (pale blue-green), emerald (deep green), heliodor (golden yellow) and morganite (pink). The biggest crystal ever found was a beryl from Malakialina, Madagascar. It measured 59 feet (18 meters) long and 11 feet (3.5 meters) across, and weighed in at a hefty 400 tons (380,000 kilograms) [sources: Banfield; Encyclopaedia Britannica; Encyclopaedia Britannica].
Silicates are only one of several elemental crystal families. Oxides (including the aforementioned corundum) contain oxygen as a negatively charged ion; phosphates pack phosphorus; borates burst with boron (B); sulfides and sulfates seethe with sulfur; and halides hold fast to chlorine and other elements from group VIIA in the periodic table [source: Banfield].
The carbonate family contains crystals rich in carbon and oxygen. Jewelers know it best for aragonite, a calcium carbonate variety that oysters use to build pearls. Aragonite can form from either geological or biological processes [sources: Banfield; Encyclopaedia Britannica].
Last but not last, deep in the Mexican state of Chihuahua there lies a limestone cavern dubbed the Cueva de los Cristales, or Cave of Crystals, shot through with soft, transparent crystals of selenium (a type of transparent gypsum) so large (in the ballpark of 30 feet or 9 meters) they dwarf human spelunkers [source: Shea].
So what's the biggest crystal anywhere in the world? It might be in the world -- literally. According to some scientists, Earth's moon-sized inner core could be one giant iron crystal [source: Broad].
Author's Note: How are crystals made?
Self-organizing systems, from ecologies to (some say) the universe itself, are in their own way as mind-bending as chaotic ones. Indeed, some have called self-organization "anti-chaos" because, while chaos is highly sensitive to initial conditions, self-organizing systems begin with a multiplicity of initial conditions and end up in virtually the same final state.
Organization and multiplicity are what crystals are all about. They are defined by order, but not order of a single kind. Multiplicities -- of morphologies, of lattices, of polyhedra, sometimes even of crystals -- are why the same pile of atoms can give us diamonds or pencil lead. There's something sublime in that.
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