Linnaeus has given an excellent review of the game mechanics and principles for Diamant, and so I don't feel justified in writing another review. However I'd like to offer some observations of my face to face (as opposed to BSW) experience of the game. This is based on about a dozen plays with three different groups of people of between four and six players. Please read Linnaeus' review first of you don't know how the game works.
Some comments after a few plays
The production values for Diamant are good. I know one person commented that he felt it was expensive for what it is and could have been fitted into a box half the size. Well, that depends on what you are looking for. You could just about play this game with a doctored set of standard playing cards and some glass chips. I feel though that the nice components are worthwhile and add greatly to the enjoyment of the game. The red and white gems are highly refractive and very tactile...most people can't resist picking up a handful of them just to feel them and clack them around in their hand. Players always want to trade in their five rubys for a diamond. This is the point of the game of course, to collect and covet gems, and so having nice ones helps with the theme. You may have noticed in the photographs that each treasure chest has a different company logo or nationality (French, German, English/American) with a word play on the colour (Yellowstone Foundation, United Ivories). These are nice touches that add to the theme and enjoyment. The cards have small witticisms here and there too - a discarded 'Indiana Jones' magazine, or half a ludo board and pieces no doubt left behind as the explorers fled (which I quite liked...the pieces in a game leaving behind the pieces of a game...).
Diamant is simple and can be explained in a couple of minutes or by running through a few cards and then restarting once everyone gets it. In my experience it plays differently with larger rather than smaller groups. With a large group there will tend to be more gems left behind on the scoring cards - for example with the maximum of eight players any card yielding up to seven gems will be left behind for the early quitters to collect, and even larger cards will tend to leave large remainders after each player gets one gem each (there is only one card yielding more than 15 gems). This ups the stakes considerably with larger groups. With smaller groups the gems will be divided out more easily and less will be left behind. So, in small groups the games are close and small gem caches really can make a difference. In large games the psychology is more intense because there are big collections at stake, but also more people to break rank and collect them (lowering the yield for each player).
This is a game of bluff and nerve, the only strategy being to try to second guess your opponent's intentions or bluff them into gullability so you can make off with the gems... Hmm, that's seven gems left behind on cards now, but NOONE is going to chicken out yet, ARE THEY? (He says, hoping to bully the rest into pressing on so he gets all of the gems on the way out for himself.)
It is true that pressing on alone can yield a large advantage if you are lucky enough to turn up a high scoring card, and this can give an unassailable lead. Or can it? Who said that this had to be a game of individuals? If one player is ahead why can't the others openly discuss tactics? " Mary has at least fifteen gems in her chest. I think some of us should turn back in case she does, so she doesn't get even more on the way out...but some of us should press on in case she keep going". Of course noone has to keep their word...
There will be some group discussion about the situation and what to do - what cards did we take out? Hmm, what are the chances of that coming up again? Some people put their fist in and then take it out - changing their mind or perhaps pretending to change their mind. Early quitters will be derided for their cowardice - "There he is, sitting in his comfortable camp whilst the rest of us press on". It's the table banter that makes this game so enjoyable, and you can only get that by playing it face to face, and preferably with people you know well. I think this is the answer to Linnaeus' challenge that there may not be enough decision making in the game to bring the theme to life. The richness comes from the player interaction, not from the game mechanics as such. You have to ham it up to have the most fun.
Downtime for players who quit early has never been an issue for us...they are too busy fiddling with their gems, deriding/encouraging the remaining players, or interfering and offering advice. And each cave is over in a few minutes anyway.
In summary you will like this game if...
1) You like Cloud 9 http://www.boardgamegeek.com/game/632 . The 'do or dare' element is present in each but the games and psychology are actually quite different and we have played both games in the same session. However they are similar in that it's not so much the game as the player interaction that adds the weight.
2) You have a group of people you know quite well, and like to spar with using bluff and reverse psychology
3) You want a fun. light game that you can use as an intro or at the end of the night, or to relax new players and get some interaction going.
I can see Diamant coming out regularly at the end of the session for quite a while to come.