Some crystals, though, de-polarize light. In most cases, they don't affect all colors of light equally. Depending on the thickness of the crystal and other factors, some colors de-polarize more than others. Here's the explanation of polarizers that I've heard for years. I'm not sure if it's true or if the accurate story is much more complicated.
Light comes in waves.
Some of them go up and down, some go side to side. They go in all different directions. Imagine a clock face on the front of a flashlight. If the flashlight were pointing at you, and were turned on, some of the light waves hitting you would be bouncing up and down, from 12 to 6. Some would bounce from 3 to 9.