The following crystals focus on enhancing learning process, and enhancing our brain functions. However, stress is also a major issue that will affect learning quality. Therefore, it will be good to use together with some Crystals for Emotional Healing.
As each of the following type of crystals has its own unique strengthen, we can't really give them a ranking.
However, my personal favorite is Labradorite. It is my first crystal pendant, and I did well during my study when I was using this. According to our customers' feedback, their kids did well for their study too; some of them even get scholarship. (But of course, we had been very hardworking too. Crystals are a "supplement" that helps us progress better with our efforts)