Diamond Rating is a metric which is invented by Kevin Broom who devised a simple general formula that works with any per-minute statistics. Formula subtracts the player’s rating per game from his rating per 40 minutes to figure out how much his per game stats undervalue his potential contributions. He then subtracts league average from the player’s per-40 minute rating and adds this amount to ensure the player is actually playing well in the minutes he does get.
The fewer minutes a player plays per game, and the better he does in those minutes, the better his Diamond Rating. Diamond Rating makes sense to eliminate players from consideration who are over 27, have more than five years experience, play more than 30 minutes per game, or played less than 250 minutes total.