The gem or diamond simply indicates a keyword with an unusually high niche value. This could either mean that the keyword has a realtively high search volume and low competition, or that it has high competition and extremely high search volume.
Here's a more detailed explanation of what the niche value (green bar) is all about:
The green bar's main function is to sort the list so that the least interesting keywords end up at the bottom of the list. The green bar (= niche value) is a function of search volume divided by competition difficulty. This means that keywords with high difficulty and low search volume have a very low niche value (because who wants to launch a huge SEO campaign to rank for a difficult keyword that gets no traffic?).
Conversely, if a keyword has a high niche value, that means it has a low competition difficulty level relative to the amount of traffic you can get for it. This could mean that it's easy to rank for, while getting a decent amount of traffic, or it could mean that it's difficult to rank for, but gets massive amounts of traffic.
In practice, the niche value usually serves mainly to eliminate the low traffic/high competition keywords. Your actual keyword selection should be based on other factors. Depending on the purpose of the website, these might include, competition difficulty, search volume, AdWords values etc.