Heals and shields are 5% stronger and increased by an additional 10% on targets below 40% health.
Unsealed Spellbook is an interesting Keystone choice on Junglers as the base Summoner CDR allows you to Smite several more times in the early game as compared to other Junglers without.
Skarner also often likes to take ghost for lane ganks in the early game, but Flash for the later portion of the game where you rely on an instant pick.
Hextech Flashtraption allows you to overcome Skarner's biggest weakness of having no "terrain scaling", meaning he is not able to find effectively ganks by being able to jump over walls like Dragon pit etc.
Skarner is able to make great use of the extra bit of Movement Speed from Magical Footwear. It also allows you to potentially complete your tier 2 boots sooner thanks to the gold savings.
If you would prefer to buy boots early for ganks, Future's Market is a good alternative option.
Cosmic Insight further doubles down on your Summoner CDR allowing you to make hard engages that much more often.
Combined with Unsealed Spellbook and Boots of Lucidity your Summoner CDR will be 40%, putting Flash on a 3min cooldown.
Unflinching combined with Unsealed Spellbook on Skarner allows you to empower your constant use of Summoner Spells even more.
After Flashing or Ghosting into a fight for a pick, the Extra bit of Slow Resist/Tenacity will allow you to pull them back even further and stick to your target.
Revitalize on Skarner massively buff your [W] and Sterak's shield making you a resilient target when diving the enemy backline for a pick.
conditioning is a solid alternative when looking to stack a lot of resists.