Final answer:
The best rock type for radiometric dating is igneous rocks, specifically those that contain the minerals zircon or hornblende.
Step-by-step explanation:
The best rock type to use for radiometric dating is igneous rocks, specifically those that contain the minerals zircon or hornblende. These minerals contain radioactive isotopes of elements such as uranium and potassium, which decay at known rates. By measuring the parent-daughter ratio of these isotopes in the rock, geologists can determine the age of the rock.