Primeval elements with long half-lives such as potassium (40K), thorium (232Th), and uranium (235U, 238U produce elements with shorter half-lives as part of the decay process. Other primeval elements such as calcium (48Ca), vanadium (50V), and nickel (58Ni) are either rare or very weakly radioactive, so are not significant when determining radioactivity of specific minerals.
Potassium typically decays via beta emission into calcium, or by electron capture and gamma ray emission to form argon. All other geologically important radioactive elements produce daughter elements which are themselves radioactive, creating a radioactive series. Although uranium and thorium have more than one possible decay mode, with potentially highly complex decay chains, both eventually decay into stable lead (Pb). The stages of the decay chains, and the half-lives of the intervening steps, significantly impact the number and energy of the gamma rays produced.
Acid igneous rocks, evaporates, and rocks formed in reducing environments all have a higher proportion of primeval elements than other rocks. The same equipment is used to measure the radioactivity of a sample to determine its physical properties, or while in a field situation detecting anomalies.
Due to strong attenuation, radioactivity is only detected in a thin surface layer, so geometry plays an important role in anomaly detection. Anomalies with a large lateral extent will be easily detected, while those with a small lateral extent may be missed if the distance to the detector is too great.
Typically, shales and sylvite contain more potassium so have high natural gamma, while limestone, sandstone, and gypsum have very little potassium so have low natural gamma. Igneous rocks with feldspar and mica weather into clays with the ability to accommodate large radioactive ions, so will also have higher natural gamma.