Why is it necessary to determine the half-life of a radioactive element?

Question

Before you can use it to date rocks....?

Answer 1

Rocks are also very particular and like to have potential suitors understand them and their temperament.  A proper suitor will get to know the intimate details about the individual rock such as the number of crevices, composition, and as you state the half-life of any radioactive elements composing them prior to asking for a date :)

Answer 2

Using radiometric dating involves comparing the percentages parent and daughter isotopes in the sample to be dated.

However, if you do not know how long it took for the parent to decay to daughter, then your percentages are useless. You just know how much has decayed, not how long it took. And without knowing how long it took, you cannot date the sample.

That's why there is a formula for decay rate as well as for half life.