Most mineral crystals grow from molten rock deep within the Earth. As a magma cools, many crystals form simultaneously and crowd into one another, producing irregular shapes. In this rock slab, quartz and feldspar crystallized to form the speckled granite.

Why are the crystals of this quartz so large and well formed? Because they grew out of mineral-rich solutions in spacious rock cavities. When solutions are completely saturated with mineral components, crystals can begin to form on rock walls, on other crystals, or even on particles of dust. They can grow quickly or slowly — or even stop — depending on changes in temperature and the concentration of the solution.