Info from RIAA.com:
“The RIAA® Gold® and Platinum® Awards program was launched in 1958 in an effort to create a standard by which to measure sales of a sound recording.
In the beginning, there was only a Gold® album award for the sale of 500,000 copies. As the industry grew, other awards were developed. The Platinum® award (1,000,000 sold) was created in 1976 and with the advent of the compact disc and the subsequent increase in sales, the Multi-Platinum™ award was created in 1984.
On March 16, 1999, the RIAA® launched the Diamond® Awards, honoring sales of 10 million copies or more of an album or single.”