"AD&D" 2nd edition and "D&D" 3rd edition divided true dragons further into three main categories: "chromatic dragons", such as green and black dragons, which are evil-aligned; "metallic dragons", such as gold and silver dragons, which are good; and neutral-aligned "gem dragons", rare creatures that possess psionic abilities. In addition, there were other sub-species of true dragons that did not fit into the three main categories. For example, mercury and steel dragons would seem to be metallic dragons, but in the "Dungeons & Dragons" world they are considered to be outside of the main family of metallic dragons because of various biological differences (though the book "Dragons of Faerûn" did list them as metallic dragons). The "lung dragons" or spirit-dragons of "Oriental Adventures" are also true dragons.