The first rock nuzzled by NASA's Mars Curiosity rover is a little more unusual than scientists expected.

Curiosity used its robot arm to touch the football-sized pyramid-shaped rock for the first time two weeks ago, and it also shot the rock dozens of times with a laser.

The results surprised scientists, who this week said 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 such as Hawaii. Those rocks are formed under high pressure, deep underground and once contained water.

Scientists do not know how old the Martian rock is.

(Above: NASA image shows where NASA's Curiosity rover aimed two different instruments to study the Jake Matijevic rock)

"It was a bit of a surprise, what we found with this rock," Curiosity scientist Ralf Gellert of the University of Guelph in Ontario, Canada, told reporters.

"It's igneous," he said, referring to rock formed from molten material. "But it seems to be a new kind of rock type that we encountered on Mars."

Curiosity arrived on Mars two months ago to learn if the most Earth-like planet in the solar system was suitable for microbial life.

Last month, Curiosity's laser was used to zap the rock and the rover analysed the pulverised material, as well as tiny pits left behind, to determine its chemical composition.

Scientists found the rock lacks magnesium and iron - elements found in igneous rock examined by previous Mars rovers Spirit and Opportunity.

The rock, named after a Jet Propulsion Laboratory rover engineer, Jake Matijevic, who died shortly after Curiosity's landing, was also rich in feldspar-like minerals, which provided clues about the rock's history.

The rover meanwhile has moved on to testing and cleaning its soil scoop.

Eventually, scientists want to funnel soil samples to Curiosity's onboard laboratory for more extensive chemical analysis.

The rover is part way to its first science target, an area known as Glenelg, which has three different types of rock intersecting.

The car-sized Curiosity rover landed inside a giant impact basin called Gale Crater, located near the Martian equator, for a two-year, $2.5bn astrobiology mission, NASA's first since the 1970s-era Viking probes.