Here's my thoughts on Skarner, and why I myself haven't put him in tier 1 yet. He belongs in low tier 2 or high tier 3 at best. He's a really good CHAMPION, and a pretty good pick, but he's an average (or slightly above average) jungler, I guess similar to Amumu, in that respect. Here's why. -

He has fast clear times - sure (please note - only if he has his blue, he's extremely prone to early invasions, without it he's just above average at best), but I feel his early game survivability and mana in the jungle are a little ridiculous. I've tried a multitude of routes, and feel it's the same story everywhere.

He basically feels like an Udyr that's considerably worse
Same ganking style (run up and hit)
Same jungling style (spam abilities and rely on blue early on)
Worse speeds
Worse sustainability
Worse survivability
Worse counter jungling
Worse answer to counter jungling
Worse ganks
Less flexible
Less of a dragon threat

He is permanently on low health in his jungle - whatever skill you take at level 2, his E's heal is pitiful, so the shield is usually better. I'd advise a change to either incorperate something with Energised from his Q (10% damage reduction or + X amount of armour), or just basically buff his base armour. He can barely complete a standard sustained (wolves to golems) route without having to pull golems or he dies.

Furthermore, his base mana seems a little low. I know you aimed at making him strong late rather than early game - but he's permanently oom, or close to oom, which makes him worse in the jungle. I'd advise reducing MP5 and boosting base mana, so the golem buff helps him out more, he can complete routes easier without running oom without golem buff, and it doesn't make his laning OP.

I feel Skarner's been neglected by Riot recently, it took two patches for them to reduce mana costs (which was definitely a step in the right direction), but he just feels like a sub-par Udyr. He doesn't do anything better and is a similar concept, a tanky melee who relies on close range encounters that builds metagolem most games. At least make them a little different, E.G., if Skarner had more speed and flexibility, sure - but Udyr is just better at everything.

You pick certain junglers for certain reasons. Like you'd pick the old Nunu for great counter jungling and early game dominance. Udyr's a good mix of everything (the Swiss army knife of the jungle), and is one of the best junglers in the game, People pick Shaco now because of his awesome ganks and counter jungling - both of which he does better than most other junglers. However, my argument is that he can't do ANYTHING better than Udyr. Udyr plays EXTREMELY similar to him and is better in EVERY aspect. There's no reason to favour Skarner in terms of jungling.

He does, however, have INSANE AoE damage, and can destroy team fights with his ultimate. He makes a great off tank and his skillset really complements it. He's a great champion, but an average, or slightly above average jungler. Definitely not one of the best junglers in the game, but some would argue (and rightfully so) his late game lets him overcome that.

He shines late game, not in the jungle.