One way we make crystals is by reshaping clear plastic objects. Basically any clear hard plastic item with flat sides can be used in this way.
Just a few examples of things that will work are leftover sprues, cocktail toothpicks (those little plastic swords), and Venetian blind wands. If you keep your eyes open you never know what cheap plastic you will spot that can be re-purposed.
An Example: Sprues
The easiest tool to use for this project will be a file or sandpaper. If you use sandpaper make sure that a sanding block, or a counter top, is used to ensure that you achieve the desired flat surface. A belt sander also works great if you happen to have one and are doing larger crystals. A saw can even be used if your crystals are really big or your saw is very small. Whatever you use to cut and shape your plastic will leave small scratches in the surface of the crystals.
Sprues have an odd wedge shape so the easiest way to use them is to remove the two 90 degree angles.
These small scratches can easily be removed by using an acrylic based floor polish. The polish will fill in the scratches and provide you with a smooth crystal like surface finish. If you prefer a rougher texture, sand all of it and not just the surfaces that were cut.