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... When the rover rolled its 899-kilogram (1,982-pound) body over the rock in May last year, the rock broke open, revealing yellow crystals of elemental sulfur: brimstone.

Although sulfates are fairly common on Mars, this represents the first time sulfur has been found on the red planet in its pure elemental form.

What's even more exciting is that the Gediz Vallis Channel, where Curiosity found the rock, is littered with rocks that look suspiciously similar to the sulfur rock before it got fortuitously crushed " suggesting that, somehow, elemental sulfur may be abundant there in some places.
Gediz Vallis Mars
Gediz Vallis channel beyond the ridge, with surrounding sulfate-bearing unit. (NASA/JPL-Caltech/ESA/University of Arizona/JHUAPL/MSSS/USGS Astrogeology Science Center)

"Finding a field of stones made of pure sulfur is like finding an oasis in the desert," said Curiosity project scientist Ashwin Vasavada of NASA's Jet Propulsion Laboratory in July.

"It shouldn't be there, so now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting." ...