I've noticed that after a recent update to my Xbox One, some achievements are considered "rare." A diamond will appear in the notification for the achievement when it is unlocked. Some of these achievements I've seen the diamond on are rather easy, and I don't get how they are considered "rare."
What determines or at what point is an achievement considered rare?
In the patch notes video for this update it was stated that an achievement is Rare if less than 10% of all players (who have started that game) have gotten it. This means an achievement can be rare, and then as more people get it, it becomes a regular achievement.
I can't find an official MS post, but this post on trueachievements.com confirms that 10% calculation.