Often referred to as a "variety" of either hematite or goethite, it is a mixture of the two minerals due to the alteration of goethite, typically found in the botryoidal habit of the "parent" goethite. It may be iridescent. While it is found in a variety of localities some of the best comes from Graves Mountain Georgia. This particular set comes from a covetous recent find dubbed "the Skittles Pocket".
Most all of these pieces are heat reactive and if you blow on them the moisture of your breath will make then change color! I did not check each piece individually but I can tell from the color shift some of these will change color nicely.