Minerals Have Too Much Helium

The shiny black specks in granite are mica. Within mica are natural zircon crystals, only a few microns in size. Helium quickly diffuses out of zircon.

If the granite is millions of years old, as commonly believed, all the helium should be gone.

However, measurements indicate that much of the helium still remains. Either the diffusion rate of the helium is not uniform, the zircon crystals are younger than believed, or both. Helium in granite is evidence that the earth is thousands of years old, not millions.

 http://www.icr.org/zircon-helium/