[thelegendaryof]

I think what you want is setmetatable. Especially the __newindex metamethod to map values as keys on creation instead of that traversing via a manual loop.

However keep in mind __newindex will only trigger if that metatable has been added and the way you're defining it right now gives it now way to trigger __newindex ever so you will need to use a virtual function like this to get it to work and keep adding it as a collection:

	function mapped_table(t) 
		local ret = setmetatable({}, {
			__newindex = function(self,k,v) 
				rawset(self,k,v) -- map index as key, return value (default)
				rawset(self,v,k) -- map value as key, return index
			end
		})
				
		for k, v in pairs(t) do
			ret[#ret+1] = v 
		end
		
		return ret
	end

	local myWhatEverMaterialTable = {}
	
	myWhatEverMaterialTable[8] = mapped_table({
		1245,
		1259872,
		25115,
		2958257,
		21587,
		2187
	})

d(myWhatEverMaterialTable[8][25115]) -- outputs index: 3
d(myWhatEverMaterialTable[8][3]) --outputs value: 25115
d(myWhatEverMaterialTable[8][666]) -- outputs nil because neither the index noir the value exists in the table

Edit:

Also never use table.insert because it 's ignoring metatables as well.
Set them manually and you're good to go (like with my function above).