Polarized light microscopy brings out incredible colors in chemicals. Have a look at Bromine under the microscope.
Holmium (a metal) looks fairly interesting by polarized light microscopy.
Have a peek at Menthol, commonly used as a local analgesic under the microscope.
Salicylic acid, a key ingredient in anti-acne products looks quite interesting under the microscope, displaying a small amount of birefringence.
Have a look at the sporophyll of Bracken under the microscope.