The history of the K/Ar-method for absolute dating of minerals and rocks is full of surprises and good guesses. The β-activity of potassium was discovered, together with that of rubidium, by J. J. Thomson as early as 1905.The history of the K/Ar-method for absolute dating

How accurate is K-Ar dating?

Potassium-argon dating is accurate from 4.3 billion years (the age of the Earth) to about 100,000 years before the present. At 100,000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40, pushing the limits of present detection devices.