Gout: monosodium urate crystals in joint fluid
400X, compensated polarized microscopy. Note that the vertically oriented crystal is yellow, while the horizontal ones are blue. This phenomenon is called "negative birefringence." Depending on the orientation of your polarizers and compensator, the colors may be reversed. This is why it's a good idea to keep a known urate control at hand. For that I use a tissue section of a gouty tophus, processed anhydrously, which is packed with the needle-like urate crystals.