Relative dating simply says one is older than the other but no age is specified.Tools in the category of radiometric dating are based on the known rates of radioactive decay of isotopes and this allow some samples to be assigned a certain age to within some accuracy.Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.
For instance, in radiocarbon dating, there really isn't a way to date something to 100,000 years or more.
A link is provided to the Wikipedia article on radiometric dating to get you more information.
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Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms." When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.
Fossils are generally found in sedimentary rock not igneous rock.
Before radiometric dating it was difficult to determine the actual age of an object.