I found that the formation of crystals is not specific to water. My hypothesis was correct: acetic acid will form crystals, but glycerol will not. I suspect this is because of the high viscosity of glycerin, whereas the viscosity of acetic acid is only slightly higher than water.

Two ideas for interesting follow-up projects have occured to me: testing many substances in an attempt to find the chemical properties that allow for crystal formation, and testing acetic acid to determine the structure of the crystals formed.

It’s really quite fun to watch the crystals grow, even with just water.  I reccommend you try it.  Water doesn’t require all the safety equipment, so you could do it at home.