As explained below, the radiocarbon date tells us when the organism was alive (not when the material was used).
This fact should always be remembered when using radiocarbon dates.
When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere.
When the rock recrystallizes it becomes impermeable to gasses again.
The amount of the isotope in the object is compared to the amount of the isotope's decay products.
For radiocarbon dating to be possible, the material must once have been part of a living organism.
This means that things like stone, metal and pottery cannot usually be directly dated by this means unless there is some organic material embedded or left as a residue.
Obviously there will usually be a loss of stable carbon too but the proportion of radiocarbon to stable carbon will reduce according to the exponential decay law: R = A exp(-T/8033) where R is C ratio of the living organism and T is the amount of time that has passed since the death of the organism.
By measuring the ratio, R, in a sample we can then calculate the age of the sample: T = -8033 ln(R/A) Both of these complications are dealt with by calibration of the radiocarbon dates against material of known age.
Once an organism dies the carbon is no longer replaced.