Rock candy formation is a perfect teaching model for mineral crystallization because the process is observable, repeatable, and edible. Dissolve 3 cups of sugar in 1 cup of boiling water (supersaturated solution). Allow to cool slightly, then suspend a seed-coated stick in the solution and leave undisturbed for 6–7 days. What grows is a monocrystalline or polycrystalline sucrose structure exhibiting real crystal habit, real nucleation behavior, and real crystal growth rate sensitivity to temperature and vibration — exactly the same physics that governs quartz growth in a hydrothermal vein. Key teachable moments: why cooling slows growth (solubility vs. temperature); why disturbing the jar produces small crystals instead of large ones (too many nucleation sites); why one side of the crystal grows faster than another (local concentration gradients). Run this demo before teaching mineral crystal systems. Students who have grown a crystal remember what crystal habit means.
Sugar Crystal Formation: Crystallography You Can Eat
A hands-on introduction to nucleation, supersaturation, and crystal habit — using sugar, water, a stick, and about a week of patience.
Educator packet
Best conducted in a classroom at stable room temperature. Avoid locations near air vents, windows with direct sun, or siblings who will eat the experiment on day three.