A user on the Geekhack forums known as T0mb3ry posted the first renders of his Carbon keyset in 2015. It's essentially an homage to number 6 on the periodic table, with a ton of cool novelty caps. It uses complementary shades of cream, dark gray, and orange. It was a huge hit when it ran on Massdrop a few months later, and it has since become one of the most desirable and valuable sets. The first round was done in the retro-style SA profile by Signature Plastics (SP), but it's also been produced in Cherry profile by GMK. In the recent second round sale of Carbon SA, demand was so high that delivery isn't expected until March 2018 at the earliest. Getting just the base set of Carbon SA or GMK will probably run you $250-300.
Update: 6 on the periodic table, not 12 as was originally written above.
MORE: Fullsize Mechanical Gaming Keyboard Price List
MORE: Tenkeyless Mechanical Gaming Keyboard Price List
MORE: History Of Mechanical Keyboards