Metallica celebrated a major career milestone in May, when their 1991 album Metallica was certified Double Diamond by the Recording Industry Association of America (RIAA). It was just the 14th album to reach that plateau, which signifies shipments of 20 million albums (or streaming equivalent units) in the U.S.

Double Diamond albums were unthinkable when the RIAA launched its gold awards program in 1958. Only one album was certified gold that year (signifying $1 million in manufacturer’s dollar volume; the criteria later changed) – the soundtrack to the film adaptation of Rodgers & Hammerstein’s stage musical Oklahoma!, starring Gordon MacRae and Shirley Jones.

Album sales grew through the 1960s and 1970s, thanks to such blockbusters as The Beatles’ Abbey Road and Carole King’s Tapestry,

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