I was talking to someone a few weeks ago about this but couldn't find information.

A list of diamond albums is here:

http://riaa.com/Gold-Best-2.cfm

Here's a blurb about the introduction of "diamond" status:

http://riaa.com/Gold-Intro.cfm

Added Sparkle
On March 16, 1999, the RIAA launched the Diamond ® Awards, honoring sales of 10 million copies or more of an album or single. Of all the artists in attendance, Sir Elton John best described the significance of the award when he said "I think this is the biggest accolade you can be given because it means your fans have gone out and bought your records. And that’s why we make records – for our public."

The list of Diamond ® titles represents some of the best and most influential recordings in history. From The Beatles to The Backstreet Boys, it is truly an audio timeline of the last 50 years. Representing all genres of music, these 58 artists and 78 tiles total more than 900 million in U.S. sales.

On commenting on the RIAA’s artist of the century, Hilary Rosen (President and CEO) stated, "These artists represent the very best of popular music and deserve the highest accolades the industry has to offer. All have been major trendsetters, as well as record-breakers. Their exceptional talent has opened doors for other artists to follow."

DebbiR