Today's Image of Mars is once again of Homestake, that mysterious vein Mars Exploration Rover Opportunity is currently inspecting. This was compiled by Stuart Atkinson from The Road to Endeavour from a collection of images taken by Opportunity's microscopic imager.
The image shows that Homestead is weathered and covered with grooves. Homestead is denser than the surrounding material, making it more resistant to weathering. But just what is Homestead made of?
While the MER team has not announced anything officially, Atkinson reports that many MER forums are abound with the theory that Homestead is made of the mineral magnesite. For more information on that particular mineral I suggest you highlight the the word "Magnesite" on my page; The Wikipedia page on Magnesite should pop up into a little window, telling you all you want to know about that mineral, and more!
This image links to Atkinson's post with this image on The Road to Endeavour.