A quick look at a wiki page produced this:
There are three levels of hardening that have been showed:
The least durable is the Armored Titan's armor-like skin, mostly because of how it spreads across the whole body.
The blue-colored hardening, which can be learnt by all Nine Titans theoretically as mentioned above, is stronger and can be applied onto different body parts like hands, feet, or nape at will.
Although having a different appearance and color, Wall Titans' hardened "wall" skin is likely at this level, for the Colossal Titan could not kick through it. It is also shown that by focusing the hardening on fewer areas will make those parts even stronger.
The strongest is the crystal-like level, which has only seen being used by Female Titan and War Hammer Titan, and it is used to contain its own user inside, for protecting purpose only. Currently, only the Jaw Titan's jaws and claws are seen to be able to penetrate through this crystal-like hardening.
About the War Hammer's appearance, it's probably caused by it's affinity to the Hardening ability.