Martian "blueberries"

This mosaic image shows spherules, or 'blueberries,' partly embedded and spread over the soil on Mars. The image is from the Mars Exploration Rover Opportunity at Meridiani Planum. The image is a mosaic of three separate images; each image is approximately 2.5 cm (1 inch) across. (Image courtesy NASA/JPL/Cornell University.)

They may be markers of former life on Mars.

One such tantalising hint was discovered by the NASA Opportunity Rover, which found small spherical hematite balls, dubbed ‘blueberries,’ in the Martian soil...

Now researchers from the University of Western Australia and University of Nebraska have found that such iron-oxide spheroids, when they appear on Earth, are formed by microbes...

On Earth, such spherical iron-oxide concretions are commonly found on beaches and deserts around the world. Similar examples to those discovered on Mars have been found in the Jurassic Navajo Sandstone near the Colorado River, Utah, where the concretions range in size from small marbles to cannonballs and consist of a hard shell of iron oxide surrounding a softer sandy interior...

Re the terrestrial ones, see my 2008 post on The creation of stone spheres.