The tiny objects, barely bigger around than huckleberries or bb shot, are stumping the rover's scientists, and they appear to have tough shells and softer interiors.
In images sent to Earth, some appear to have been cracked open - perhaps by an ancient impact - but how they formed in a Martian region now believed to be more than 3 billion years old is just as mysterious the planet itself.
Stephen S. Squyres, Opportunity's chief scientist, discussed the objects Tuesday at the annual meeting of the American Geophysical Union in Moscone Center and will report on them more formally Wednesday.
The rover landed on Mars in January 2004 on what was to be a three-month mission to seek signs of ancient water on the planet. It has now traveled 22 miles and has found plenty of highly suggestive evidence that water existed on Mars long ago.
One major piece of evidence that Opportunity discovered early in the mission were tiny spheres in varied sizes that Squyres has nicknamed "blueberries."
Those objects varied in size and contained the iron mineral hematite, which clearly forms in water. They lay in a crater where the sand was heavy in sulfates, so the water there would have been a weaker version of sulfuric acid, Squyres and his colleagues reported at the time.
Now Opportunity's scientists at NASA's Jet Propulsion Laboratory in Pasadena, exploring the giant Martian crater Endeavour by remote control, have found what seem to be hundreds of the "newberries" lying on rocky outcrops of one part of the crater's rim.
Here, unlike the acid environment of the "blueberries," the "newberries" lie in clay rock that has formed in a far more benign kind of water "in an earlier time when the water was not acid, and possibly suitable for life," Squyres said Tuesday.
The tiny spheres are a big mystery, he said. They might have formed from unknown minerals that precipitated out of ancient sandstone, or they might be volcanic, belched out of some ancient eruption, or they might have been sculpted long ago by Martian winds, he suggested.
"Clearly this is very ancient rock," he said of the area, perhaps 3 billion years old. A spacecraft in orbit around Mars has detected "unambiguous" infrared signals of the clays on the surface there, he noted.
"We're trying to keep an open mind about the newberries," he said.
There are a good six more months of exploring before the next Martian winter slows things down for a time until a 10th year may come for Opportunity.
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