Nicatonic Acid, or Vitamin B3 looks interesting under the microscope and displays some birefringence.
Salicylic acid, a key ingredient in anti-acne products looks quite interesting under the microscope, displaying a small amount of birefringence.
Urea (or carbamide) displays some cool birefringence under the microscope, have a peek!
Sulfur is an abundant element found in all living cells, what does it look like under the microscope?
Caffeine, the popular additive in all sorts of beverages (including many of my favorites) looks stunning by polarized light microscopy!