As I played Diamant my first time, I wasn’t very amazed: quite simple rules, maybe a good party game but nothing special to offer for 3 players. However, after few games with different numbers of players it turned out that Diamant is like cheap beer: not so elegant but still great with larger group and not to be taken so seriously.
After all, the game is very simple: Every player is a wannabe-Indiana Jones, who explore a cave trying to recover a treasure as large as possible and then escaping before the cave collapses killing everyone still inside. Every round a card is turn. After that it’s effects are resolved. If it has gems those are divided equally among the players, meaning that less players are still in the cave, more there is to get. If the card represents a hazard (explosions, snakes, scorpions…) it does nothing unless it is the second hazard of the same type, when it collapses the cave killing everyone still inside. After five cave explorations players seem to notice that cave-exploring is very dangerous to health and nerves and quit their job, when the player who has got the most gems out of the cave wins.
Luckily you can escape any time you like. After every card, players are given a choice between escaping cowardly and advancing deeper to the cave. Everyone reveals their choices simultaneously and those coward enough to run keep all the gems got this far, while the brave ones reveal the next card, risking the chance of second hazard. Obviously you try to do the opposite that everyone else does: when you're alone no one is there to share the gems with you.
As the game focuses on guessing each other's thoughts, it's important how interesting it is to guess your opponents thoughts. And to answer right away, it is interesting. It also balances quite nicely weights of luck and skill for such a quick game: better and more experienced has an advantage, but those newcomers who are “just playing for fun” still have their shots. As stated before, Diamant shines with larger groups. Larger group doesn't even slow the game, as all the actions are done simultaneously.
Diamant has it's problems too. One is that most of the time decision whether to continue or not is trivial. Anyway it takes the same time as the important decision points and this can be quite annoying. This means large chunk of the gaming time goes automatically between the important parts. Other problem is how long this kind of filler can last before it looses it's amusement value. I can't give an exact answer because of lack of testing, but I can fairly confidently say it is still playable after 20 plays. And because of it's nature, most of those games will be played with different groups. So even if you will get fed up with it, your fellow gamers probably won't.
So is Diamant worth buying or not? It depends on whether you have a need for another filler game or not. If you own something like High Society or 6 nimmt! and still enjoy playing it Diamant won't be very likely worth it's price yet. However, if you have a hole in your shelf for a small snack between those larger games this might be the game for you. Now you must just question yourself do you like guessing.