A grain of the mineral zircon, found some years ago from the Jack Hills of Western Australia, is now thought to be the oldest known piece of the Earth’s crust.
Atom-probe tomography has settled earlier uncertainties about the age of 4.4Gyrs.
The blue grain of zircon shown above is less than 0.1mm in length and has a grey outer region known to have formed 1Gyr after the blue core.
JW Valley et al (Nature Geoscience 7, 219-223, February 23, 2014) measured the uranium – lead isotopes of this grain by atom-probe tomography to determine the age. This gave an atom-scale map of the distribution of the isotopes. It means the Earth’s crust formed just after the Earth – Moon system was established (click to enlarge the time-scale above).
Zircon is a mineral made up of zirconium silicate, ZrSiO4 and comes in different colours. You can buy it from Amazon.
(posting from an idea by Jerry Hayes)