From GeekDad, the art and science behind crystals:
Crystals are cool because how they look to our eye is how their atoms are arranged. For instance, one project the kids did was inspired by mini-marshmallow-and-toothpick models of carbon atoms when they’re arranged to make diamonds.
We also painted pictures on black construction paper using a super-saturated solution of Epsom salt and water (a variation on the salt crystal activity found in my book Discover the Desert). The paintings come out glittery, although we didn’t get the snowflake-like results of the version found at Anne Marie Helmenstine’s great chemistry blog on About.com.
But the biggest project of the day was growing Salt Crystal Gardens and Salt Crystal Trees. The magic ingredient in these fluffy creations is Mrs. Stewart’s Bluing, a non-toxic laundry additive that can be found in grocery stores in my area. “Gardens” are grown on a base of cut-up cubes of kitchen sponge. The “trees” above use cardboard toilet paper tubes as a growing surface.