Attack on titan Hardening Hardness

Hardening Hardness
It seems that some of the nine titans can harden their body. However, I noticed a difference in their durability and hardness. So, I created a kind of classification of titan crystals.

This hardened material is shown to be so strong that not even the Colossus Titan could kick through it, instead having to kick through a man-made gate. Hardened materials on Titans are shown to be penetrated by Thunder Spears, sheer strength (presumably by another Titan), or the power of the Founding Titan. Titan hardening performed around humans, has only been shown to be broken by the Jaw Titan's jaw or claws, along with the power of the Founding Titan, with things such as Thunder Spears merely bouncing off of the Titan crystals. This skill has shown to be easier to learn based on the Titan, such as Eren's Attack Titan not having the skill to harden until a special serum allowed him to harden almost instantly after ingesting it.