Final answer:
Volcanic rocks and minerals high in potassium-40 are the best samples for K/Ar dating, as the ratio of decayed argon-40 to potassium-40 helps determine the rock's age.
Step-by-step explanation:
Volcanic rocks and minerals which contain a high amount of the isotope potassium-40 (K-40) make the best samples for the K/Ar dating method. This is because K-40 decays to argon-40 (Ar-40) with a half-life of 1.25 billion years, and this decay rate is a fundamental principle in geochronology for determining the age of rocks. When a rock sample containing K-40 is crushed, the liberated Ar-40 gas is measured. Assessing the ratio of Ar-40 to K-40 in the sample can provide the age of the rock since K-40 started decaying after the rock formed. This method is particularly useful for dating igneous and metamorphic rocks, and by using this approach, scientists can estimate the lower limit for the earth's age as it is assumed that the planet is older than the oldest rocks on its surface.