The Difference Between Rocks and Minerals

Remember when we made "homemade geodes" using alum? 

The Alum that you buy has been crushed into a powder. In this experiment, you will reconstruct its natural form, which is crystalline. You will see how different the structure of a mineral is compared to a rock."

A string is suspended in a saturated solution of alum.
The weights are to keep the suspended into the solution.
This is left for from a few hours to a few days.
After several days, as you can see, we had alum crystals "growing" on the string, but they were even larger on the side of the glass...perhaps there was something wrong with my string. Perhaps it was coated with something that prevented the crystals from collecting there. Despite this, we did get to see the alum crystals on the side of the glass.

"If you compare the alum crystals to rocks, you should notice a strong difference. Minerals make up most of what we find in the earth. Sometimes, the minerals can be found in their pure form. When that happens, the minerals usually have a sharp, geometric pattern. Most of the time, however, the minerals are not found in their pure form. Instead they are found in the form of rocks, which are usually made up of more than one kind of mineral."

source: Exploring Creation with General Science by Dr. Jay L. Wile