Ok, so my title for this post is perhaps a teensy bit of a misleading tease, but while the rock in question might not be the most interesting rock on our planet (I might rank ancient Archean crust or carbonaceous chondrite meteorites higher) it is arguably the most interesting rock from the Moon.

During the Apollo 17 mission in 1972 the astronaut and geologist Harrison Schmitt collected a small piece of lunar material, barely 5 centimeters across, that was both colorful (an unusual feature on the lunar surface) and later found to be about 4.2 billion years old from radioisotope measurements. 

That means that this sample, now with its official designation of Troctolite 76535, cooled and crystallized barely 300 million years after the formation of the Moon. That's critically important, it has helped us begin to piece together the Moon's history - after all, without a measure of its oldest materials we might be stuck just guessing at its age.

In addition, 76535 has apparently never been badly shocked, smashed or otherwise vigorously bumped around by any further geophysical processes or by stuff whacking the lunar surface in the subsequent few billion years. That's quite an accomplishment in the lunar environment, if a rock can have accomplishments.

That's not to say that it's had an entirely clean slate, just not a lot of alteration. For example, researchers have been able to see evidence of two distinct magnetized patterns in the rock. One pattern was likely set during the rock's formation some 30 miles down in the lunar crust, the other is orientated differently and shows evidence that the earlier magnetic structure was partially erased by heating. The implication being that an impact event dug 76535 out from its deep origin, and warmed it enough for a new magnetic fingerprint to impose itself. And what was causing the magnetic imprint? The Moon itself - revealing something about its deep geophysical history, and the likelihood of an internal dynamo, generating that magnetic field.

Troctolite 76535, this tiny piece of our sister world, continues to provide scientists with insights to the lunar past - from whether there was a magma ocean, to the detailed differentiation of the lunar interior, and even the use of radionuclide dating techniques.

So if you are a rock enthusiast, this one is for you.

(You can look at all the Apollo lunar samples here)