Thallium's name captures the fresh green of the spectral line that revealed it, thallos evoking the first green shoot of spring, and it belongs with caesium, rubidium and indium among the elements named for the colours of their light. Its darker reputation, however, is as a poison: thallium salts are colourless, odourless and tasteless, and their effects, including hair loss and nerve damage, can mimic other illnesses, which earned thallium a grim renown as the poisoner's poison and a place in criminal history and detective fiction, notably Agatha Christie's novel The Pale Horse, whose accurate description of thallium poisoning is said to have helped diagnose real cases.
The element has no ancient or alchemical tradition, being a spectroscopic discovery of the 1860s. The same heart-imaging use mentioned above, thallium-201 stress testing, exploits a biological quirk: the body's potassium-handling machinery cannot easily distinguish thallium ions from potassium, so the radioactive isotope is taken up by healthy heart muscle in rough proportion to its blood supply, letting cardiologists read areas of reduced flow directly from the resulting scan, a genuinely useful medical inversion of the same chemical mimicry that makes thallium so toxic in the first place.