In your kitchen you start a three-minute egg timer and a 60-minute hourglass simultaneously and then leave.
You return a short while later to find the hourglass fully discharged but not the egg timer!
Radioactive crystals make and lose helium To understand the two hourglasses, you need to know what goes on in tiny radioactive forms around it.
Other minerals, such as quartz and feldspar, crystallize around the biotite to complete the rock.
However, are all other rocks in the earth's crust also susceptible to "contamination" by excess emanating from the mantle?
The nuclei of the uranium atoms (embedded throughout the zircon) decay through a series of intermediate elements to eventually become nuclei of lead.
Many of the intermediate nuclei emit particles, which are nuclei of helium atoms.
They use uranium, lead, and helium changing within a common mineral.
I'd like to take this opportunity to explain the hourglass illustration more fully, because it clarifies some very encouraging scientific evidence that supports the Biblical timescale of Earth history -- six thousand years, not billions of years.