Diamonds is a trick-taking card game where players collect Diamonds - not cards of that suit, mind you, but rather actual "Diamond Crystals" (acrylic crystals) included in the game.
What makes the game of Diamonds unique from other trick-taking card games is that when you cannot follow suit you get a "Suit Action" based on what suit you do play. Suit Actions are also awarded by the winner of each trick, as well as at the end of a full Round of play.
Suit Actions will allow players to take Diamond Crystals from the Supply, moving them to their Showroom (where they may score 1 point) or to their Vault (where they will score 2 points). The Vault is a secure area, but the Showroom is vulnerable to thievery by the other players.
The player with the most points in Diamond Crystals at the end of the game wins!