Curiosity used its robot arm to touch at a football-sized pyramid-shaped rock for the first time two weeks ago. It also shot the rock dozens of times with a laser.
The results surprised scientists. They said Thursday that it is not like other rocks seen on Mars. It has more sodium and potassium.
Scientist Edward Stolper said the rock is more like rare volcanic rocks seen on Earth in places like Hawaii. Those rocks are formed under high pressure, deep underground and once contained water.
Scientists don't know how old the Martian rock is.