The word "lithium" is derived from the Greek word for stone, "lithos", because it was first discovered in stone. Lithium is one of several alkali metals discovered and named in the early 1800s. The other two, sodium and potassium were also named after the materials that were being studied when they were discovered.
Potassium's name is derived from the English word "potash", a term for plant ashes that is no longer in common use. Sodium's name comes from "soda", a word of possibly Arabic origins that refers to the ashes of a particularly sodium rich plant, the saltwort.
The other two alkali metals had been discovered by an English scientist named Davy, using his newly developed method of electrolysis. Lithium was discovered by the Swedish scientist Johan August Arfwedson 8 years later, in 1817. Arfwedson was studying a mineral called petalite. The mineral had been previously described by a Brazilian scientist named José Andrada, but Andrada had not analysed the mineral's individual components.
While studying the mineral, Arfwedson found that it was composed of silica and alumina. It also contained, at 3% by weight, an unknown alkaline substance. Attempts to draw it out using tartaric acid failed, proving that the substance could not be potassium or magnesium. Arfwedson concluded that this substance must be a new alkali metal. Berzelius, his assistant, notes that it was named "lithium" because it was discovered in a mineral, whereas the two other new metals were discovered in botanical sources.