…everyone’s talking about. It does seem odd. There’s what looks like a dark circular patch on the ground in the original view, and then that doughnut rock appears above it in the next frame:

It’s apparently unusually high in magnesium & sulphur. It presumably fell off the rover’s wheel, and is surely just a coincidence that it appeared above the circular patch, but it does add to the sense of mystery…
Report: http://www.mirror.co.uk/news/world-news/mars-rock-looks-like-jelly-3039561