The exposed vein--seen here in an image taken from the rover's panoramic camera--is apparently made of gypsum, deposited by water. Called "Homestake," the vein 0.4 to 0.8 inches long (about the width of a human thumb) and 16 to 20 inches long, NASA said in a press release.
Opportunity, which has been on Mars for almost eight years, has been exploring the area located along the west rim of the Endeavour Crater since August. The exposures were taken on November 7.
After identifying the visible deposit in November, Opportunity's Microscopic Imager and Alpha Particle X-ray Spectrometer on the rover's arm examined the vein and "the spectrometer identified plentiful calcium and sulfur in a ratio pointing to relatively pure calcium sulfate," NASA said.