1991 Metallica "The Black Album"

This is the highest any metal album as every come into the mainstream world and popularity. The album was the only one ever to reach the level of Diamond level by selling over 15 million copies. The Black Album was the first album to branch away from the original style of thrash and speed metal to replace it with a heaver and slower tempo. This concept album was first of its kind for Metallica and sparked several other bands to tweak their styles. The Black Album showed the world that change could be good even in a subject such as metal. This was the biggest jewel that Metallica has ever and probably will ever receive in its career.