Love crystals were an item Harry Mudd discovered while on the planet Sirius IX in 2269. He, in turn, sold the crystals to several thousand of the planet's inhabitants, who experienced adverse effects from them.
Escaping subsequent charges on Sirius, Mudd escaped to Motherlode, where he attempted to sell his remaining love crystals to the miners for three hundred credits per crystal. There he was found by Spock of the USS Enterprise, and was taken into custody aboard the starship. On the Enterprise, Mudd talked Head Nurse Christine Chapel into taking a crystal from him to use on Spock. He also broke several crystals near an air system grille to aid in his escape aboard the shuttlecraft NCC-1701⁄12.
The crystals worked by breaking the crystal and applying the liquid within to one's skin, and then touching another person. Rubbing the liquid potion on the skin could cause temporary dizziness. They were said to cause romantic feelings amongst men and women, and warm fellowship among members of the same gender. To Harry Mudd's own surprise, the crystals actually had a temporary effect on the Enterprise crew. Spock fell briefly in love with Nurse Chapel, while the crew of the Enterprise herself became intoxicated by the love potion's fumes through the air ducts. Montgomery Scott and M'Ress briefly flirted with each other, the transporter officer was too busy dancing with a female officer to beam anyone up, and Kirk and Spock acted like good buddies.
Less fortunately for Mudd's possible hopes of an honest pursuit of interstellar commerce, the potion had a rebound effect once its main effect wore off; that meant the crystals would be a poor buy at even half-price. (TAS: "Mudd's Passion")