The golf ball-sized rock is believed to be a meteorite fallen from the Red Planet’s sky.
Such meteorites are common on Earth and have been found on Mars before, but this is the first to be examined by Curiosity’s laser-firing spectrometer.
The tool identified the Egg Rock’s composition as iron and nickel.
Scientists who operate Curiosity noticed the rock in images captured by the rover at the end of October.
“The dark, smooth and lustrous aspect of this target, and its sort of spherical shape attracted the attention of some MSL scientists when we received the Mastcam images at the new location,” said ChemCam team member Pierre-Yves Meslin.
The researchers explained that iron meteorites typically form when asteroids heat up and the molten metal inside them sinks to the core.
The meteorites provide records of many different asteroids which broke up, with fragments falling to Earth and Mars, the researchers said.
They can shed light on how Mars’ environment has affected them, in comparison with the effect of Earth’s environment on similar meteorites.
Curiosity was launched five years ago this month is currently making its way up Mount Sharp, in an attempt to learn how the planet’s environmental conditions have changed over time.
It’s believed the region once offered conditions which could have harboured microbial life.