Radiometric dating is based on Sexprofiles dating
This is why crystals are good for radiometric dating: the atoms in a crystal are extremely efficiently packed, and it's very difficult to get anything into a crystal such as a contaminant by any means short of destroying the crystal and re-growing it anew.
The oldest crystals on Earth that were formed on Earth are zircon crystals, and are approximately 4.1 billion years old.
Uranium eventually decays into lead, and lead does not normally occur in zircon, except as the radioactive decay product of uranium.
Therefore, by measuring the ratio of lead to uranium in a crystal of zircon, you can tell how much uranium there originally was in the crystal, which, combined with knowing the radioactive half-life of uranium, tells you how old the crystal is.
The process of radiogenic dating is usually done using some sort of mass spectrometer.
A mass spectrometer is an instrument that separates atoms based on their mass.
We have dated meteorites using Rb-Sr, Sm-Nd, Pb-Pb, Re-Os, and Lu-Hf isotope systems and have obtained very similar ages.
The fact that the age we calculate is reproducible for these different systems is significant.
We know it is accurate because radiometric dating is based on the radioactive decay of unstable isotopes.Obviously, if the substance you are measuring is contaminated, then all you know is the age since contamination, or worse, you don't know anything, because the contamination might be in the opposite direction - suppose, for example, you're looking at radio carbon (carbon 14, which is produced in the atmosphere by cosmic rays, and which decays into nitrogen).Since you are exposed to the atmosphere and contain carbon, if you get oils from your skin onto an archeological artifact, then attempting to date it using radio carbon will fail because you are measuring the age of the oils on your skin, not the age of the artifact.For an example of how geologists use radiometric dating, read on: A geologist can pick up a rock from a mountainside somewhere, and bring it back to the lab, and separate out the individual minerals that compose the rock.They can then look at a single mineral, and using an instrument called a mass spectrometer, they can measure the amount of parent and the amount of daughter in that mineral.
There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.