The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.
Zircon has a high hardness (7.5) which makes it resistant to mechanical weathering, and it is also very resistant to chemical weathering. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.
The USGS based their study on minerals that naturally occur in rocks and have half-lives of 700 million to 100 billion years.
These dating techniques, known as radiometric dating, are firmly grounded in physics and are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to re-homogenize its radioactive elements.
The dating equation used for K-Ar is: Carbon Dating Radiocarbon dating is different than the other methods of dating because it cannot be used to directly date rocks, but can only be used to date organic material produced by once living organisms.
That is a question that cuts to the heart of it all.
An event like metamorphism could heat the crystal to the point where Pb will become mobile.