Types of uranium dating
The reason that I trust the accuracy of the age that we have determined for the earth (~4.56 billion years) is that we have been able to obtain a very similar result using many different isotopic systems.Most estimates of the age of the earth come from dating meteorites that have fallen to Earth (because we think that they formed in our solar nebula very close to the time that the earth formed).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 call the original, unstable isotope (Uranium) the "parent", and the product of decay (Lead) the "daughter".From careful physics and chemistry experiments, we know that parents turn into daughters at a very consistent, predictable rate.
I do think that radiometric dating is an accurate way to date the earth, although I am a geochronologist so I have my biases.
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.
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.