This is the battery cover of the HTC Diamond.
As the name suggests, 'Diamond', the phone has a 'diamond' characteristics to it, ie/ it has a shapy bulky diamond-like cut back cover, as can be seen in the picture. Though it may be seen as a fashion icon, (and perhaps sleek) the pointy back cover really is not very ideal for every day usage of a phone (as most people lie their phone on the back, and this will cause some sort of friction unless you have a cover). Ultimately, the back cover will be scratched and wear down, (well mostly all the pointy parts sticking up from the cover will be eroding away), and sooner than expected, the phone will look rugged, because of the back.
This is the replacement for such a design mistake by HTC designers. The extended battery would be very useful, as the normal battery will wear down quickly too, due to its bright, high res screen the battery will be drained, and the battery is not large enough to accomodate every day needs.