Here is an interesting science experiment that creates a Christmas ornament to go along with the Jesus' birth story of Angels and Shepherds: Grow a Crystal Angel Ornament.
Crystals are grown on colorful pipe cleaners (chenille stems) from a saturated borax solution. Borax crystals grow faster than sugar or salt crystals. Crystals will start forming about two hours after shape is hung in glass of solution. This is probably not quick enough for an in-class project, but could be done at a special event or at-home.
For each cup of water (boiling) you will need about 3 tablespoons of borax (found with laundry supplies; dissolve as much in the hot water as you can). Make an angel shape from pipe cleaners and suspend the angel in a glass of hot borax solution and crystals will form as solution cools.