Rutile, Brookite
Thumbnail, 1.8 x 1.8 x 1.4 cm
Magnet Cove, Hot Spring Co., Arkansas, USA

A sharp, cyclic-twinned brookite crystal replaced by lustrous rutile. Old classic material. Much better in person. 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). Ex. Carlton Davis Collection.