Which Mars rover discovered 'blueberries' (hematite spheres) on Mars?

NASA's Opportunity rover discovered Mars's famous “blueberries,” small hematite-rich spheres found across Meridiani Planum. The rover photographed and analyzed them soon after landing in January 2004.

The spheres earned their nickname because they appeared bluish-gray against Mars's widespread rusty-red soil. Opportunity's instruments showed that their iron mineral component was dominated by hematite, a form of iron oxide. Their rounded shapes and distribution indicated that many formed as concretions within rock and were later exposed as the surrounding material eroded.

The discovery mattered because hematite and the geological setting pointed to an ancient watery environment. It did not prove that life existed on Mars, but it provided strong evidence that liquid water had once influenced the region. The blueberries are sometimes confused with ordinary pebbles or with discoveries made by Spirit, Curiosity, or Perseverance; this specific finding is strongly associated with Opportunity.