I am not sure but I think the oldest one we have found is only a little over 4 GA (billion years)
The oldest known
rocks (assemblages of minerals) are from the Slave Lake region of Canada, and are indeed just over 4 GA. Bowring, S. A. & Williams, I. S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada, Contrib. Mineral. Petrol. v134 #1 pp 3-16; three rocks have ages of 4.0020.004 billion years (sample SAB91-63), 4.0120.006 billion years (sample SAB91-37), and 4.0310.003 billion years (sample SAB94-134). The Tera-Wasserberg concordia-discordia plot for sample SAB94-134 is at
http://i2.photobucket.com/...10/JonF/SlaveLake_SAB94-134.png.
The oldest known terrestrial minerals are zircons from Jack Hills in Australia, found in sedimentary rocks (so the rocks are not that old).
Wilde SA, Valley JW, Peck WH and Graham CM (2001) Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago. Nature. 409: 175-178. The zircons are 4.4040.008 GA. Lots more on these zircons at
Zircons Are Forever.
The date often used of 4.5 GA is using other evidence such as dating moon rocks, meteors, etc that are objects as old as the earth since they formed form during the same stellar phenomenon that formed the earth.
Indeed the dates of many moon rocks and meteors are around 4.5 GA, and that is good evidence that the Earth is about that old. (See
Part II. Radio-isotopic Dating; at the end are several tables of such ages reproduced from Dalrymple's
"The Age of the Earth").
But the 4.55 GA age of the Earth is derived from a lead-lead isochron analysis of
almost solely terrestrial sources. Lead-lead isochrons are unlike other isochrons in that they are not "anchored"; the intercepts of the isochron line with the axes is meaningless, and you need to obtain an "anchor point" for the isochron line from something other than the lead content of your samples. We need to figure out the primordial lead isotope ratios of the Solar System; and that we get from the
Canyon Diablo meteorite. This meteorite is primordial and contains essentially zero uranium (less than 10 ppb). So its current lead composition is also primordial to within a lot better than one percent. This anchors the isochron, and then terrestrial samples define the slope of the isochron, which determines the age; 4.55 billion years.
There's a lot more to it than this; Dalrymple devotes an entire chapter to it, and that's not the really technical version. In particular, the primordiality of the Canyon Diablo meteorite has been checked many ways and passed all tests.