I tend to be wary of generalized recommendations, since I've found there can be quite a lot of individual variation in people's reactions to stones, and also variation in stones of the same type, but, that said-
I've found that black star diopside affects me in a way that is not so much a concentration of attention, but an amplification of attention (and sometimes intention), and most noticeably, improves memory retention- such that I actually avoid wearing it when I am attending events where there will be much trivia that I don't think will be worth remembering in a specially reinforced way.
When in a situation with stimuli which I previously judged to be important to pay attention to, though, that is not a problem.
I have three jewelry pieces of black star diopside, and I have noticed this same effect with all of them, though with a bit of comparative variation in strength between them.