credit: SnowCrystals.com; image source; larger image
A Snow Crystal Primer
You can learn about snowflake physics at SnowCrystals.com by Kenneth Libbrecht at Caltech.
Although stalactites result from a chemical reaction and icicles from a physical change, they share a common form. For time-lapse photos of a simulation of stalactite formation, with a model stalagmite growing underneath, see Stalactite.