Sapphire decks are dominated by items and the powerful Flaversham draw engine, while Amethyst is built around the Merlin/Madam Mim bounce package that was introduced as early as set two
It really helps that the reward of Marchesa, Resolute Monarch has one of the rarest effects in all of Magic in being able to remove all counters from a permanent
B : A high quality common
It enters tapped unless it's one of your first two lands, and it taps to produce a single white mana
Naban, Dean of Iteration One of the first typal cards on the list, Naban, Dean of Iteration is the epitome of niche but powerful
Obviously stealing planeswalkers can be very powerful, especially if you can ultimate them right away, but Callidus Assassins flash, as well as the fact that it destroys the creature rather than having your opponent get it back when it leaves the battlefield, are significant as well