Caffeine

Crystals of caffeine.

Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug.

In humans, caffeine acts as a central nervous system stimulant, temporarily warding off drowsiness and restoring alertness. It is the world's most widely consumed psychoactive drug, but, unlike many other psychoactive substances, it is both legal and unregulated in nearly all parts of the world.

I keep a stock solution of caffeine on the bench but at the stock concentration it slowly crystallises, forming long thin delicate crystals.