Various methods have been devised to determine this initial or common Pb, but all involve making unprovable assumptions.
Zircon does incorporate initial Pb when it crystallizes. It cannot be proven that the Pb in apparently cogenetic U- or Th-free minerals is only initial Pb, and that it is identical to the initial Pb in the mineral being dated.
Zircon occurs in many colors, including reddish brown, yellow, green, blue, gray and colorless.
In geological settings, the development of pink, red, and purple zircon occurs after hundreds of millions of years, if the crystal has sufficient trace elements to produce color centers.
U-Pb radioisotope dating is now the absolute dating method of first choice among geochronologists, especially using the mineral zircon.