Thallium was discovered in 1861 and isolated in 1862. It occurs in crooksite, lorandite, and hutchinsonite and is present in pyrites.

When exposed to air, thallium's metallic luster develops a blue-gray tinge. It's soft and malleable and can be cut with a knife. Natural thallium is a mixture of two isotopes; a total 25 isotopic forms exist.

Thallium and its compounds are toxic and suspected carcinogens. They were once used as rodenticides and ant killers, but their household use was outlawed in the U.S. in 1975. They're also used in photocells, infrared optical materials, and specialty glass.