How Do Crystals Form?

How do crystals form step by step

Crystals form step by step, and the first step is the hardest. In nucleation, a few atoms line up in a cluster stable enough to survive, often on a speck of dust or a rough rock surface. In growth, more atoms attach to that seed layer by layer, and the slowest-growing faces end up as the flat faces you see. Growth stops when the supply runs out, the space fills or conditions change. NASA's page on crystallization covers the same process.

How crystals form from magma

Crystals form from magma as molten rock cools and its atoms slow down enough to lock in place. Slow cooling deep underground gives large crystals, which is why granite has grains you can see. Fast cooling at the surface gives tiny crystals, as in basalt, or none at all: obsidian is lava that cooled too fast to crystallize, so it is glass. The last, water-rich melt of a cooling granite can form pegmatite, where quartz, feldspar, tourmaline and spodumene grow into very large crystals.

How crystals form from water

Most crystals in a rock shop form from water, either hot and under pressure or cool and slow. Hot water moving through cracks carries dissolved silica, and as it cools, quartz grows on the walls of veins and cavities. That is how most quartz points and amethyst clusters grew. When salty water evaporates, halite and gypsum crystallize out, which is how selenite and desert rose form in drying basins. Cool groundwater seeping into hollows builds geodes over very long spans of time.

How crystals form from vapor and solid rock

Some crystals form from vapor, skipping the liquid stage entirely. Sulfur crystals grow around volcanic vents as hot gas cools, and snowflakes are ice crystals that grew straight from water vapor. Other crystals grow inside solid rock without melting it. During metamorphism, heat and pressure rearrange atoms into new minerals, which is how garnet and kyanite crystals appear in schist.

Why crystals have flat faces

Crystals have flat faces because their atoms stack in a fixed pattern, and the outside shape follows the inside. In 1669 the Danish scientist Nicolas Steno noticed that the angles between matching faces on quartz crystals are always the same, whatever the size of the crystal. Mineralogists now sort crystals into seven crystal systems by symmetry. The overall shape, called habit, also depends on growing conditions, which is why fluorite grows as cubes in one place and octahedra in another.

How long does it take for crystals to form

How long it takes for crystals to form ranges from seconds to millions of years, depending on how fast the raw material arrives.

Seconds to minutes: snowflakes, frost, and lab-grown bismuth, whose rainbow colors come from a thin oxide film.
Hours to weeks: salt, sugar or alum crystals grown at home from a saturated solution.
Weeks to months: synthetic quartz grown in high-pressure vessels for electronics.
Thousands to millions of years: natural quartz points, amethyst, geodes and the giant selenite beams of Mexico's Naica mine, fed by slow-moving fluids.

Can you grow crystals at home

You can grow crystals at home with salt, sugar or alum and a jar of hot water. Dissolve as much as the water will take, let it cool slowly, and hang a string or a small seed crystal in it. The rule that governs a quartz vein applies here too: slow cooling and a steady supply give bigger, cleaner crystals. Skip copper sulfate kits around kids, since those bright blue crystals are toxic.