What it was
A crystal smaller than a grain of sand outlasts the rock around it. The rock crumbles, the grain washes free, and younger stone traps it like a pebble in wet cement. This is how a tiny zircon can keep a very old clue.
Zircon is a tough mineral that grows as melted rock cools. Heat and chemical wear destroy many softer minerals, but zircon can hold on. You could lose almost a whole ancient landscape and still find some of these grains tucked into later rock.
When and where
The Jack Hills lie in Western Australia. Some zircons from there are about 4.4 billion years old, among the oldest surviving pieces of Earth itself. Meteorites can hold older stuff, but that material formed beyond Earth. These grains grew here.
How big
Many grains span just 100 to 400 micrometers. A micrometer is a thousandth of a millimeter. Think small sand grains, with room inside for atoms that tell both an age and a clue to the water around their vanished parent rock.
How we know
Growing zircon takes in uranium but shuts most lead out. Uranium turns slowly into lead, starting a clock. The mix of oxygen isotopes, atoms with different weights, tells another story. Some grains point to source rock that met liquid water before it melted and grew the zircon.
Still a mystery
Scientists still argue about how much of early Earth was cool and wet. Claims of an ancient magnetic field in these grains are disputed too. A crystal can take on a magnetic mark later. The age of the stone need not be the age of its magnetic memory.