NASA scientists said this week the rover uncovered "one of the coolest things Opportunity has found in a very long time:" a dark, basketball-sized rock known as "Marquette Island."

According to NASA the Marquette Island rock is a coarse-grained rock that indicates it cooled slowly from molten rock, allowing crystals time to grow. Such composition suggests it originated deep in the crust, not at the surface where it would cool quicker and have finer-grained texture, NASA stated.

The rock's interior contains more magnesium than in typical Spirit has studied, according to NASA. Researchers are determining whether it might represent the precursor rock altered long ago by sulfuric acid to become the sulfate-rich sandstone bedrock that blankets the region of Mars that Opportunity is exploring, NASA stated.

Opportunity's rock abrasion tool -- which was built by Honeybee Robotics Spacecraft Mechanisms -- was used to grind away some of the rock's surface and expose the interior. This was the 38th rock Opportunity has ground into, and one of the hardest, NASA stated.