They make slices of zircon 30 micrometers thick and mount them on glass. These slices are examined to see if they had consistent growth or if they had disturbances; if they are simple or complex, in granite or metamorphic rock.
Scientists observe how light reflects from the grains of zircon, and in detail, how it is transmitted through them.
Scientists use this method to date rocks that formed from between 1 million to 4.5 billion years ago, and they estimate the Earth is 4.543 billion years old.
The oldest and most reliable method they use is called Uranium-lead (U-Pb) dating.
Zircon also concentrates Uranium (You) and (although less so) Thorium (Th) and excludes lead (Pb), which means it has a very high You/Pb ratio.
Also, zircons are magnetic so scientists can separate the finest pieces with magnets.