Spiky Crystals

Grow crystals in your fridge with epsom salt and hot water!
Chemistry crystals (featured above) - Instructables
Cup of Quick Crystal Needles - About.com

You can also explore evaporation by growing crystals outdoors! Check out Exploratorium's Spiky Sun Crystals!

To incorporate this into a fossil unit, grow crystals on a sponge to investigate permineralization, visit Layers of Learning!