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.
Have a look at Cardiac muscle under the microscope :).
Benzoic Acid is a simple aromatic carboxylic acid, with a crystalline structure that lights up in wonderful colors.