Basically, they took chunks of what they already knew was a very old (~3.6 billion years old) rock, ground them up, separated out the zircon grains, and dated m

Basically, they took chunks of what they already knew was a very old (~3.6 billion years old) rock, ground them up, separated out the zircon grains, and dated many (100's to 1000's) of individual zircon grains.