Pb (lead-204), which is neither unstable nor radiogenic.
Troilite (Fe S) from iron-rich meteorites fits the bill: its present ratio of uranium to lead is so tiny that either the solar system and indeed the universe is many many times older than cosmologists think, or, given the long half-life (4.5 billion years) of U, there can hardly have been any uranium in the meteorites to start with, and so its decay can hardly have affected the lead isotope ratios of these meteorites.
You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth.
Second, zircons are durable and chemically inert, able to resist chemical weathering and even high-grade metamorphism up to about 900 °C.
Zircons, then, are relatively immune to the problems that make isochron U-Pb dating so difficult.
Uranium can and often does substitute for the element yttrium, whereas lead cannot, making xenotime suitable for radiometric dating.