ToolMaterial Balancing/Opinion Question

[TrekkieCub314]

So I'm wondering what went through the collective minds over at Mojang regarding how materials are balanced.  The number of uses an item has seems to be roughly proportional to their real-life counterparts' Mohs hardness, as is their strength against an entity.  However, I'm trying to figure out how they balanced Efficiency, as well as Enchantability.  Enchantability seems to be a rough approximation of how much magic-related myth surrounds a material (although I would expect diamond to be higher than iron, which is not the case), but Efficiency isn't making much sense to me.  Wood is 2.0, Stone is 4.0, Iron 6, Diamond 8, Gold 12.  Did they just increment the numbers by two based on the hardness of the material and then decide to make Gold max just to make gold tools useful, or is there a reason based on real-life materials?

 

I ask, of course, because I'm looking to add custom tools and I'm trying to figure out balancing.

[Draco18s]

Diamond enchantability is lower than iron, presumably, because it's also the most efficient and most durable already.

 

As for efficiency, yes, it's just an increase with each tier with gold being super fast and super enchantable so that it's actually useful (and still no one uses it).