In particular, biological materials alter igneous rocks in well-understood ways.

CNN: Curiosity opening Martian frontier?

And that has Prof Stolper and the rest of the rover science team leaning (with a lot of caveats) towards a classification that sits on a well-established sequence of alkaline igneous rocks - one that goes by the name "mugearite".

BBC: Cosmic coincidence on the road to Glenelg

Prof Stolper compared the signatures with a catalogue containing thousands of Earth rocks, and determined the nearest match to be an igneous type, the formation of which he likened to the production of colonial apple jack liquor.

BBC: Nasa's Curiosity rover finds 'unusual rock'