A moon rock discovered on Earth could fetch about $US500,000 ($A706,800) when it goes on auction.

A 5.4kg lunar meteorite discovered in northwest Africa last year is up for auction by Boston-based RR Auction and will be subject to online bidding that runs from Thursday until October 18.

RR described it as "one of the most important meteorites available for acquisition anywhere in the world today" and one of the biggest pieces of the moon put up for sale.

The rock classified as NWA 11789, also known as "Buagaba", was found last year in a remote area of Mauritania but probably plunged to Earth thousands of years ago.

The meteorite is actually composed of six fragments that fit together like a puzzle.

Most lunar meteorites found are the size of a walnut or golf ball, said Geoff Notkin, chief executive of Aerolite Meteorites, which is selling the rock.

"As soon as we saw this, we knew it was extraordinarily unusual," he said. "This is close to a once in a lifetime find."

It is also one of the few known lunar meteorites with what experts call "partial fusion crust" caused by the tremendous heat that sears the rock as it descends through the atmosphere.

Another thing that makes it different from most meteorites is that it is "unpaired".

Sometimes different pieces of the same meteorite are discovered at different times, and those examples are known as "paired".

An "unpaired" meteorite is more desirable to collectors and perhaps more valuable to science.