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The so-called Genesis rock was found by lunar module pilots David Scott and James Irwin when they dug into the slope of Spur crater, on the flank of the Apennine Mountains.
They were there on the second "moon safari", travelling for the first time in a custom-built lunar rover vehicle.
The rover, which looks like a four-wheeled Jeep, has enabled the astronauts to spend more time away from the lunar module than ever before, and to go several miles away from the lunar lander, Falcon.
There was a small setback when the front-wheel steering failed soon after setting off for the first time yesterday.
"It's just sporty driving," commented Scott. "I've got to keep my eye on the road every second."
The fault was fixed in time for a longer drive today.
The astronauts have so far spent 18 hours on the surface of the moon in three major moonwalks.
They have collected 169 lbs (76.8kg) of moon rock from 12 different sites including Hadley Rille, an ancient channel believed to have been carved by torrents of flowing lava.
But the highlight has been today's discovery of the ancient crystallised rock, believed to be about 4,500 million years old - dating back to the time the Moon itself was formed.
Live on TV
All the astronauts' movements were followed in a live colour television transmission with unprecedented images of the Moon's highlands.
They included the 15,000 ft (4,500 metres) high Mount Hadley, towering over the landing site of the lunar capsule at the foot of the Apennine mountain range.
For the first time, the cameras were controlled from Earth, freeing up the astronauts to describe and explore what they were seeing.
At the end of today's seven-hour expedition, flight director Gerald Griffin said, "I think without doubt we've just witnessed the greatest day of scientific exploration that we've ever seen in the space programme - possibly of all time."
The experiments carried out by Apollo 15 were the most complex yet, and were originally planned for the cancelled Apollo 20 space mission.
The last three Apollo missions have now been cut, and there are just two more flights scheduled to the Moon, the last due to take off next year.
The Genesis Rock proved to be a chunk of anorthosite, part of the original lunar crust and older than any Moon rock previously found.
This one rock helped revolutionise ideas about lunar formation, and gave us new insights into the age of the solar system.
For the first time, the astronauts were extensively trained as geologists and could make scientific observations, both on the surface and from orbit.
After Apollo 15, there were just two more manned missions to the Moon, with the final manned lunar landing, Apollo 17, completed in December 1972.
However, in January 2004 the US President, George Bush, announced a new programme for lunar exploration, saying American astronauts would return to the Moon by 2020 as the launching point for missions further into space.
James Irwin resigned from Nasa in 1972 to found a religious organisation, High Flight Foundation, and led two expeditions to Mount Ararat in search of Noah's Ark. He died in 1991, aged 61.
David Scott retired from Nasa in 1977 to found Scott Science and Technology.
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