Hello!
Let me start off by saying, this is my FIRST guide, so there are mistakes to be found.
But anyways, it's a guide about Skarner, a great tanky DPS champion. This is my very own build, with which I've had amazing experience. So far, out of about 30 games played, only 2 were lost.
In this guide I will try to explain as good as I can, everything about runes, masteries, spells etc...

Here is a picture of my match history:
In addition, there is a ranked game with 16/0/10 score.

I find Skarner to be a very good champion, so there are more pros than there are cons.

Pros:

Great durability with amazing damage
Great chaser
Great pusher
Hard to harass
Hard to focus in fights
Easy diver
Good solo laner

Skarner has great durability, and his damage in mid to late game is really amazing. Movement speed from Crystalline Exoskeleton is great for chasing, combined with either the suppress from Impale or slow from Crystal Slash. Skarner is also hard to harass. Enemies might harass you to 50% HP, but using Fracture will heal you in no-time. Due to Fracture, he is also a good solo lane champion.


Cons:
Currently, I can't find any bad things with Skarner.

For runes, I only take Armor Penetration and Durability. Therefore, I use these runes:
9x Greater Mark of Desolation Greater Mark of Desolation
9x Greater Seal of Resilience
9x Greater Glyph of Warding
3x Greater Quintessence of Fortitude

But at times, I use 9x Greater Seal of Clarity and 3x Greater Quintessence of Desolation Greater Quintessence of Desolation, for more Mana regeneration, especially if I am taking a solo lane.

I start off with a Regrowth Pendant, which I turn into Philosopher's Stone the first time I go back to the base. Next, I buy Boots of Speed, and a Sheen. For the extra Gold/10 I buy a Heart of Gold. After that, I buy Mercury's Treads, and Phage, aiming for Trinity Force, without buying Zeal before. Now, I have my first core item. For my next item, I take Atma's Impaler, because of the armor and the great passive, which raises my attack damage to about 200. Now, I need some more life steal and spell vamp, so I buy Hextech Gunblade, which I place on my first slot in inventory, for easier active usage. Now I have all three core items. Next item depends on my score. If I'm not doing too well, I buy Randuin's Omen for more armor, or Banshee's Veil if they have more magic damage, and I sell my Philosopher's Stone Philosopher's Stone, but if I'm doing good, I still sell Philosopher's Stone Philosopher's Stone, and buy Black Cleaver. At this point, the game is pretty much over, but if not, I go towards more attack damage, or attack speed.

I go for basic 0/9/21 masteries, for the extra mana regeneration, more experience, cooldowns and I take advanced Flash.

I take Flash and Ignite as my spells. Flash is great for chasing and getting out of combat. I will often use it to flash to an enemy, using Impale on him and pulling him back to my team, or if I get focused hard in a fight, I will try to flash over a wall, which will in most cases prevent me from dying.
Ignite is very optional. I tend to use it, because I play in a premade team a lot, and in that team there is only one Ignite, apart from mine.

Ghost would be a good option as well. Using Ghost with will allow you to pull your enemy further away, since your speed is faster.