Scientists commonly use carbon-14 (¹⁴C) to date wooden artifacts.
The ¹⁴C:¹²C ratio is constant in living plants. When they die, the ¹⁴C decays, so the ratio changes. The half-life for the for the decay is 5730 a, so scientists can work backwards and calculate how long ago the plant died. The method is good for samples up to 50 000 a old.
²³⁸U gradually decays to lead. The U:Pb ratio can give the age of a rock that is between 100 000 a and 1 200 000 a old.
³⁸Ar can date certain rocks that are between 200 000 a and 4 000 000 a old.
⁴⁰K decays to ⁴⁰Ar. Geologists can use K:Ar dating for rocks up to four billion years old.