A sharp, cyclic- twinned brookite crystal replaced by lustrous rutile! Old classic material! Mike Howard had an article in Rocks and Minerals a few years back that talked about these crystals, in considerable detail. Basically, any "brookite" that is a pseudo-hexagonal bipyramid (not twinned) and that is bigger than a quarter-inch is virtually always replaced by rutile (this can be recognized by striations, that sometimes create a "moire effect," on the surface.. You can also often spot a hint of the red internal reflections that characterize rutile, and the color is a little different from the black lustrous brookite crystals.) I could be wrong and this one could be an exception - a very large brookite dipyramid. The striations I do see tell me it is a rutile.